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

[Jan Schultke via cfe-commits]

eisenwave wrote:

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.

Maybe that means it should have been in a separate file after all, or is there some clever way to keep the comment?

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