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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM, but I think the next steps are two-fold: add a second `Sema` class for something else (perhaps OpenMP?) and add a base class for accessing commonly used functionality (`getASTContext()`, `Diag()`, etc).

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