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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > Release note to say we enable this in C2Y?
> 
> I'm not opposed, but we've always supported this feature. Similar to `i` and `j` suffixes, I didn't add a release for imaginary constants which are in the same boat (extensions we've supported forever but are now C2y features). WDYT?

There is a minor value (at least for messaging!) for a release note that says "we looked at implementing N3370, saw it was implemented, so added sufficient testing to validate it, so now you can use it without getting GNU-ext warnings in C"


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