[clang] [C2y] Add test coverage for WG14 N3370 (PR #115054)

[via cfe-commits]

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

Changes lgtm.

I don’t really have a strong opinion on the release note: on the one hand,  writing one for this does feel a bit like we’d be writing a release note for C2y itself rather than for Clang, but on the other, I guess adding something like ‘Clang now diagnoses case ranges as a Clang extension rather than a GNU extension’ would be an option. Iirc I added a similar release note when I changed some variadic macro warnings.

https://github.com/llvm/llvm-project/pull/115054