[clang] [clang][Modules] Complete the implementation of P2615: Meaningful exports (PR #194201)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

> This introduces a fair bit of duplication though; are we fine with that?

Yes. Regular tests and conformance test can evolve independently (the former — when implementation changes, the latter — when wording changes), so it's always a mistake if a particular aspect of implementation behavior is tested only by conformance tests.

> These should be there already.

I meant that they should be included in the CWG2443 test, which you did.

https://github.com/llvm/llvm-project/pull/194201