[Modules] structs declared in different modules conflict with each other

[davidstone]

Given the following two translation units

export module a;

export template<typename>
struct holder {
};

export struct a {
	holder<struct foo> m;
};

export module b;

import a;

struct b {
	holder<struct foo> m;
};

When compiled with

clang++ -std=c++20 -x c++-module --precompile -c a.cpp -o a.pcm
clang++ -std=c++20 -fmodule-file=a=a.pcm --precompile -x c++-module -c b.cpp -o b.pcm

Causes clang to fail with

b.cpp:6:16: error: declaration of 'foo' in module b follows declaration in module a
        holder<struct foo> m;
                      ^
a.cpp:8:16: note: previous declaration is here
        holder<struct foo> m;
                      ^
1 error generated.

I'm not 100% sure of what the standard mandates here.

[llvmbot]

@llvm/issue-subscribers-clang-modules

[ChuanqiXu9]

This is somewhat similar with #61465.