[clang] [clang] Document CWG3129 as resolved (PR #206335)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll approved this pull request.

> The one problem with putting it into cwg31xx.cpp is that I had to get rid of the // cxx98-14-no-diagnostics comment; otherwise this test just wouldn't pass.

This is totally fine. Special needs I mentioned before are scoped to RUN lines.

You should change the title of the PR to something along the lines of "Add test for CWG3129".

Let me know when it's ready to be merged.

https://github.com/llvm/llvm-project/pull/206335