[clang] [Sema] Warn about omitting deprecated enumerator in switch (PR #138562)

[Aaron Ballman via cfe-commits]

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

Please add a release note to `clang/docs/ReleaseNotes.rst` so users know about the new functionality and flag.

Otherwise, LGTM!

https://github.com/llvm/llvm-project/pull/138562