[C++ Modules] Exporting Redeclaration within language linkage

[ChuanqiXu9]

This is discussed in WG21's mailing list. Although this is not formally merged into the standard, I think it is fine to work on it already.

Reproducer:

export module mod;
extern "C++" void func();
export extern "C++" {
    void func();
}

Clang now will reject it due to [module.intefcae] p6. But we think the rejection may not be necessary. Since the intention of [module.intefcae] p6 is to avoid the name conflicting. But the entities in language linkage are always attached to global module. So we don't need to worry about the name conflicting here.

And both MSVC and GCC accept it: https://godbolt.org/z/74sTe4nP6

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Chuanqi Xu (ChuanqiXu9)

This is discussed in WG21's mailing list. Although this is not formally merged into the standard, I think it is fine to work on it already.

Reproducer:

export module mod;
extern "C++" void func();
export extern "C++" {
    void func();
}

Clang now will reject it due to [[module.intefcae] p6](https://eel.is/c++draft/module.interface#6). But we think the rejection may not be necessary. Since the intention of [[module.intefcae] p6](https://eel.is/c++draft/module.interface#6) is to avoid the name conflicting. But the entities in language linkage are always attached to global module. So we don't need to worry about the name conflicting here.