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

[Sam McCall via cfe-commits]

https://github.com/sam-mccall approved this pull request.

This direction looks good to me, mostly I just have nits.

The one major change I'd consider before landing is to find a way to avoid the verbosity regression cor3ntin mentions for `Diag()` and friends. Notational regressions within Sema itself may be as important as those of its clients, as it's so much code.

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