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

[Yanzuo Liu via cfe-commits]

zwuis wrote:

Does this PR implement [CWG2443](https://wg21.link/cwg2443)? If yes, please add/move some tests to clang/test/CXX/drs, then run clang/www/make_cxx_dr_status.

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