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

This patch moves OpenACC parts of `Sema` into a separate class
`SemaOpenACC` that is placed in a separate header `Sema/SemaOpenACC.h`.
This patch is intended to be a model of factoring things out of `Sema`,
so I picked a small OpenACC part.

Goals are the following:
1) Split `Sema` into manageable parts.
2) Make dependencies between parts visible.
3) Improve Clang development cycle by avoiding recompiling unrelated
parts of the compiler.
4) Avoid compile-time regressions.
5) Avoid notational regressions in the code that uses Sema.