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

[Aaron Ballman via cfe-commits]

AaronBallman 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?

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