[clang] [clang][NFC] Add test for CWG issues about linkage in cross-TU context (PR #113736)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@ChuanqiXu9 Can you take a quick look to make sure I'm not tripping over any modules lines in those tests?
This is the PR I promised to pull you in almost 2 weeks ago (sorry for the delay 😄).

https://github.com/llvm/llvm-project/pull/113736