compiler memory leak analyzing ZIR instruction for @extern builtin

[andrewrk]

Zig Version: 0.10.0-dev.3483+c6ad6e62f

$ stage2/bin/zig build -p stage3 -Denable-llvm -Dskip-install-lib-files -Dforce-gpa
$ stage3/bin/zig build-exe ../test/standalone/cat/main.zig 
error(gpa): memory address 0x7f09e34c50d0 leaked: 
/home/andy/dev/zig/lib/std/mem/Allocator.zig:517:40: 0x300c7e2 in dupeZ__anon_7207 (zig)
    const new_buf = try allocator.alloc(T, m.len + 1);
                                       ^
/home/andy/dev/zig/src/Sema.zig:19694:39: 0x376d1b5 in zirBuiltinExtern (zig)
    new_decl.name = try sema.gpa.dupeZ(u8, options.name);
                                      ^
/home/andy/dev/zig/src/Sema.zig:934:72: 0x3596696 in analyzeBodyInner (zig)
                    .builtin_extern        => try sema.zirBuiltinExtern(     block, extended),
                                                                       ^
/home/andy/dev/zig/src/Sema.zig:603:30: 0x358b69b in analyzeBody (zig)
    _ = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                             ^