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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Also, having to use absolute line numbers in expected directives sucks. I hope I wouldn't need to change any comments at the top of CWG1884 test. At some point I'll make `-verify` markers work for cross-TU scenarios.

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