[clang] [clang] Introduce `SemaOpenMP` (PR #88642)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

I intentionally split formatting changes into a separate commit if reviewers want to look at changes without formatting noise.

If, given the volume of changes here, there is an appetite to remove `OpenMP` and `OMP` from names inside `SemaOpenMP` right in this patch, I can do that. I did that for HLSL and OpenACC, but those were an order of magnitude smaller patches that this one.

https://github.com/llvm/llvm-project/pull/88642