[clang] [clang] Introduce `SemaX86` (PR #93098)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

@RKSimon Not at the moment. The current goal is to reduce amount of declarations in `Sema`, and target-specific functions are easy to move out and, importantly, teach where new ones should be placed. No functional changes intended.

I don't know much about our CodeGen, so I'm not proposing anything myself. That said, if there's an appetite for something of this sort there, and someone knowledgeable of CodeGen is willing to guide me and help get stakeholders agree, I can do the work.

https://github.com/llvm/llvm-project/pull/93098