[clang] Introduce `SemaOpenMP` (#88642)

This patch moves OpenMP-related entities out of `Sema` to a newly
created `SemaOpenMP` class. This is a part of the effort to split `Sema`
up, and follows the recent example of CUDA, OpenACC, SYCL, HLSL.
Additional context can be found in
https://github.com/llvm/llvm-project/pull/82217,
https://github.com/llvm/llvm-project/pull/84184,
https://github.com/llvm/llvm-project/pull/87634.