[clang] [clang] Factor out OpenACC part of `Sema` (PR #84184)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

I added convenience functions to access `ASTContext` and other widely used facilities of `Sema` to `SemaOpenACC`. I intentionally didn't go full base class approach, saving this option for the future.

https://github.com/llvm/llvm-project/pull/84184