C ABI tests failing; self-hosted llvm backend is still missing C ABI implementation code

[andrewrk]

Zig Version: 0.10.0-dev.3291+4624c8189

Can be reproduced by cd into test/standalone/c_abi and then zig build.

thread 410936 panic: TODO
/home/andy/Downloads/zig/src/codegen/llvm.zig:9345:41: 0x3721fee in codegen.llvm.ParamTypeIterator.nextInner (zig)
                                        @panic("TODO");
                                        ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:9230:25: 0x3712636 in codegen.llvm.ParamTypeIterator.next (zig)
        return nextInner(it, ty);
                        ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:2838:23: 0x351c788 in codegen.llvm.DeclGen.lowerTypeFn (zig)
        while (it.next()) |lowering| switch (lowering) {
                      ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:2798:38: 0x333c15e in codegen.llvm.DeclGen.lowerTypeInner (zig)
            .Fn => return lowerTypeFn(dg, t),
                                     ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:2433:43: 0x3336771 in codegen.llvm.DeclGen.lowerType (zig)
        const llvm_ty = try lowerTypeInner(dg, t);
                                          ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:2251:41: 0x332ed2b in codegen.llvm.DeclGen.resolveLlvmFunction (zig)
        const fn_type = try dg.lowerType(zig_fn_type);
                                        ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:3190:46: 0x3331ff9 in codegen.llvm.DeclGen.lowerValue (zig)
                return dg.resolveLlvmFunction(fn_decl_index);
                                             ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:3963:48: 0x3956191 in codegen.llvm.FuncGen.resolveInst (zig)
        const llvm_val = try self.dg.lowerValue(.{ .ty = ty, .val = val });
                                               ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:4227:45: 0x37391f8 in codegen.llvm.FuncGen.airCall (zig)
        const llvm_fn = try self.resolveInst(pl_op.operand);
                                            ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:4117:55: 0x37169b8 in codegen.llvm.FuncGen.genBody (zig)
                .call              => try self.airCall(inst, .Auto),
                                                      ^
/home/andy/Downloads/zig/src/codegen/llvm.zig:972:19: 0x3710edd in codegen.llvm.Object.updateFunc (zig)
        fg.genBody(air.getMainBody()) catch |err| switch (err) {
                  ^
/home/andy/Downloads/zig/src/link/Elf.zig:2377:74: 0x34cc537 in link.Elf.updateFunc (zig)
        if (self.llvm_object) |llvm_object| return llvm_object.updateFunc(module, func, air, liveness);
                                                                         ^
/home/andy/Downloads/zig/src/link.zig:509:77: 0x32e7a3d in link.File.updateFunc (zig)
            .elf   => return @fieldParentPtr(Elf,   "base", base).updateFunc(module, func, air, liveness),
                                                                            ^
/home/andy/Downloads/zig/src/Module.zig:4140:41: 0x32c7d45 in Module.ensureFuncBodyAnalyzed (zig)
            mod.comp.bin_file.updateFunc(mod, func, air, liveness) catch |err| switch (err) {
                                        ^
/home/andy/Downloads/zig/src/Compilation.zig:2979:42: 0x2ff81aa in Compilation.processOneJob (zig)
            module.ensureFuncBodyAnalyzed(func) catch |err| switch (err) {
                                         ^
/home/andy/Downloads/zig/src/Compilation.zig:2911:30: 0x2fe1d1d in Compilation.performAllTheWork (zig)
            try processOneJob(comp, work_item);
                             ^
/home/andy/Downloads/zig/src/Compilation.zig:2251:31: 0x2fda1f3 in Compilation.update (zig)
    try comp.performAllTheWork(main_progress_node);
                              ^
/home/andy/Downloads/zig/src/main.zig:3305:20: 0x2f452bf in updateModule (zig)
    try comp.update();
                   ^
/home/andy/Downloads/zig/src/main.zig:2994:17: 0x2f0d085 in buildOutputType (zig)
    updateModule(gpa, comp, hook) catch |err| switch (err) {
                ^
/home/andy/Downloads/zig/src/main.zig:225:31: 0x2ef0fd6 in mainArgs (zig)
        return buildOutputType(gpa, arena, args, .zig_test);
                              ^
/home/andy/Downloads/zig/src/main.zig:174:20: 0x2ef0024 in main (zig)
    return mainArgs(gpa, arena, args);
                   ^
/home/andy/Downloads/zig/lib/std/start.zig:580:37: 0x2ef28ef in std.start.main (zig)
            const result = root.main() catch |err| {
                                    ^
error: test...
error: The following command terminated unexpectedly:
/home/andy/Downloads/zig/build-release/stage2/bin/zig test /home/andy/Downloads/zig/test/stage1/c_abi/main.zig /home/andy/Downloads/zig/test/stage1/c_abi/zig-cache/o/d0cfaaa82bdd8732d3090be48e387c1f/cfuncs.o -lc --cache-dir /home/andy/Downloads/zig/test/stage1/c_abi/zig-cache --global-cache-dir /home/andy/.cache/zig --name test --enable-cache 

Don't forget to re-enable the test case when fixing this.