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

[Fangrui Song via cfe-commits]

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

bazel-6.3.2 build --config=generic_clang @llvm-project//clang

build passes with this refactoring. Thanks

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