[clang] [diag] Silence `-Wfixed-enum-extension` in C23 (PR #68060)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I believe this needs a release note as well (see docs/ReleaseNotes.rst), else this LGTM.

https://github.com/llvm/llvm-project/pull/68060