segfault when testing a second copy of the std lib

[andrewrk]

Steps to reproduce:

1. Build zig from source without using -Dskip-install-lib-files. There are now 2 copies of lib/, one in the source tree, and one in zig-out/lib.
2. Run std lib tests on the source tree lib/ without passing --zig-lib-dir.

$ stage3/bin/zig test ../lib/std/std.zig -femit-docs=$(pwd)/blah -fno-emit-bin
Semantic Analysis [1537] openZ... Segmentation fault at address 0x4b
/home/andy/dev/zig/lib/std/hash_map.zig:1231:13: 0x764ac15 in getPtr (zig)
            }
            ^
/home/andy/dev/zig/src/Sema.zig:13654:34: 0x738b137 in zirClosureGet (zig)
        if (scope.captures.getPtr(inst_data.inst)) |tv| {
                                 ^
/home/andy/dev/zig/src/Sema.zig:725:68: 0x71bb839 in analyzeBodyInner (zig)
            .closure_get                  => try sema.zirClosureGet(block, inst),
                                                                   ^
/home/andy/dev/zig/src/Sema.zig:609:30: 0x71b9da8 in analyzeBody (zig)
    _ = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                             ^
/home/andy/dev/zig/src/Sema.zig:6038:33: 0x7648522 in analyzeCall (zig)
                sema.analyzeBody(&child_block, fn_info.body) catch |err| switch (err) {
                                ^
/home/andy/dev/zig/src/Sema.zig:5574:28: 0x738af05 in zirCall (zig)
    return sema.analyzeCall(block, func, func_src, call_src, modifier, ensure_result_used, resolved_args, bound_arg_src);
                           ^
/home/andy/dev/zig/src/Sema.zig:724:62: 0x71bb75b in analyzeBodyInner (zig)
            .call                         => try sema.zirCall(block, inst),
                                                             ^
/home/andy/dev/zig/src/Sema.zig:626:45: 0x7036778 in analyzeBodyBreak (zig)
    const break_inst = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                                            ^
/home/andy/dev/zig/src/Sema.zig:1252:62: 0x71cac38 in analyzeBodyInner (zig)
                const break_data = (try sema.analyzeBodyBreak(block, inline_body)) orelse
                                                             ^
/home/andy/dev/zig/src/Sema.zig:609:30: 0x71b9da8 in analyzeBody (zig)
    _ = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                             ^
/home/andy/dev/zig/src/Sema.zig:6038:33: 0x7648522 in analyzeCall (zig)
                sema.analyzeBody(&child_block, fn_info.body) catch |err| switch (err) {
                                ^
/home/andy/dev/zig/src/Sema.zig:5574:28: 0x738af05 in zirCall (zig)
    return sema.analyzeCall(block, func, func_src, call_src, modifier, ensure_result_used, resolved_args, bound_arg_src);
                           ^
/home/andy/dev/zig/src/Sema.zig:724:62: 0x71bb75b in analyzeBodyInner (zig)
            .call                         => try sema.zirCall(block, inst),
                                                             ^
/home/andy/dev/zig/src/Sema.zig:626:45: 0x7036778 in analyzeBodyBreak (zig)
    const break_inst = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                                            ^
/home/andy/dev/zig/src/Sema.zig:1252:62: 0x71cac38 in analyzeBodyInner (zig)
                const break_data = (try sema.analyzeBodyBreak(block, inline_body)) orelse
                                                             ^
/home/andy/dev/zig/src/Sema.zig:609:30: 0x71b9da8 in analyzeBody (zig)
    _ = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                             ^
/home/andy/dev/zig/src/Sema.zig:6038:33: 0x7648522 in analyzeCall (zig)
                sema.analyzeBody(&child_block, fn_info.body) catch |err| switch (err) {
                                ^
/home/andy/dev/zig/src/Sema.zig:5574:28: 0x738af05 in zirCall (zig)
    return sema.analyzeCall(block, func, func_src, call_src, modifier, ensure_result_used, resolved_args, bound_arg_src);
                           ^
/home/andy/dev/zig/src/Sema.zig:724:62: 0x71bb75b in analyzeBodyInner (zig)
            .call                         => try sema.zirCall(block, inst),
                                                             ^
/home/andy/dev/zig/src/Sema.zig:626:45: 0x7036778 in analyzeBodyBreak (zig)
    const break_inst = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                                            ^
/home/andy/dev/zig/src/Sema.zig:591:50: 0x7642575 in resolveBody (zig)
    const break_data = (try sema.analyzeBodyBreak(block, body)) orelse
                                                 ^
/home/andy/dev/zig/src/Sema.zig:7605:42: 0x7655abf in resolveGenericBody (zig)
        const uncasted = sema.resolveBody(block, body, func_inst) catch |err| break :err err;
                                         ^
/home/andy/dev/zig/src/Sema.zig:7544:59: 0x7394fa7 in zirFunc (zig)
            const ret_ty_val = try sema.resolveGenericBody(block, ret_ty_src, ret_ty_body, inst, Type.type, "return type must be comptime known");
                                                          ^
/home/andy/dev/zig/src/Sema.zig:759:62: 0x71bd650 in analyzeBodyInner (zig)
            .func                         => try sema.zirFunc(block, inst, false),
                                                             ^
/home/andy/dev/zig/src/Sema.zig:626:45: 0x7036778 in analyzeBodyBreak (zig)
    const break_inst = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                                            ^
/home/andy/dev/zig/src/Sema.zig:591:50: 0x7642575 in resolveBody (zig)
    const break_data = (try sema.analyzeBodyBreak(block, body)) orelse
                                                 ^
/home/andy/dev/zig/src/Sema.zig:6565:53: 0x7831137 in instantiateGenericCall (zig)
        const new_func_inst = child_sema.resolveBody(&child_block, fn_info.param_body, fn_info.param_body_inst) catch |err| {
                                                    ^
/home/andy/dev/zig/src/Sema.zig:5783:40: 0x7644914 in analyzeCall (zig)
        if (sema.instantiateGenericCall(
                                       ^
/home/andy/dev/zig/src/Sema.zig:5574:28: 0x738af05 in zirCall (zig)
    return sema.analyzeCall(block, func, func_src, call_src, modifier, ensure_result_used, resolved_args, bound_arg_src);
                           ^
/home/andy/dev/zig/src/Sema.zig:724:62: 0x71bb75b in analyzeBodyInner (zig)
            .call                         => try sema.zirCall(block, inst),
                                                             ^
/home/andy/dev/zig/src/Sema.zig:626:45: 0x7036778 in analyzeBodyBreak (zig)
    const break_inst = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                                            ^
/home/andy/dev/zig/src/Sema.zig:591:50: 0x7642575 in resolveBody (zig)
    const break_data = (try sema.analyzeBodyBreak(block, body)) orelse
                                                 ^
/home/andy/dev/zig/src/Sema.zig:5571:56: 0x738ad09 in zirCall (zig)
        resolved_args[arg_index] = try sema.resolveBody(block, args_body[arg_start..arg_end], inst);
                                                       ^
/home/andy/dev/zig/src/Sema.zig:724:62: 0x71bb75b in analyzeBodyInner (zig)
            .call                         => try sema.zirCall(block, inst),
                                                             ^
/home/andy/dev/zig/src/Sema.zig:609:30: 0x71b9da8 in analyzeBody (zig)
    _ = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                             ^
/home/andy/dev/zig/src/Module.zig:5534:21: 0x702344c in analyzeFnBody (zig)
    sema.analyzeBody(&inner_block, fn_info.body) catch |err| switch (err) {
                    ^
/home/andy/dev/zig/src/Module.zig:4245:40: 0x6e4b4a4 in ensureFuncBodyAnalyzed (zig)
            var air = mod.analyzeFnBody(func, sema_arena) catch |err| switch (err) {
                                       ^
/home/andy/dev/zig/src/Compilation.zig:2977:42: 0x6e49a12 in processOneJob (zig)
            module.ensureFuncBodyAnalyzed(func) catch |err| switch (err) {
                                         ^
/home/andy/dev/zig/src/Compilation.zig:2915:30: 0x6d5a6ba in performAllTheWork (zig)
            try processOneJob(comp, work_item);
                             ^
/home/andy/dev/zig/src/Compilation.zig:2255:31: 0x6d571cc in update (zig)
    try comp.performAllTheWork(main_progress_node);
                              ^
/home/andy/dev/zig/src/main.zig:3334:20: 0x6d81306 in updateModule (zig)
    try comp.update();
                   ^
/home/andy/dev/zig/src/main.zig:3008:17: 0x6b2b4b4 in buildOutputType (zig)
    updateModule(gpa, comp, hook) catch |err| switch (err) {
                ^
/home/andy/dev/zig/src/main.zig:236:31: 0x6ab189b in mainArgs (zig)
        return buildOutputType(gpa, arena, args, .zig_test);
                              ^
/home/andy/dev/zig/src/stage1.zig:48:24: 0x6ab0c62 in main (zig)
        stage2.mainArgs(gpa, arena, args) catch unreachable;
                       ^
Aborted (core dumped)