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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Thank you for this factoring! Personally, I think this is a good model to go with for factoring functionality out of Sema and adding a tiny bit of layering to this part of the compiler (full disclosure: Vlad and I worked on this design offline). However, I added several other folks from the community to make sure there's some wider agreement on the approach as a general model.

The basic idea is to split mostly self-contained functionality off into their own classes to reduce the size of Sema.h, have better organization of concerns, make it more explicit where there are semantic connections between language technologies (e.g., where HLSL uses ObjC functionality, etc), and hopefully to help reduce incremental compile times for the project.

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