Draft extracting CGAtomic to LLVMFrontend

[Meinersbur]

Create new module LLVMFrontendAtomic as helper for emitting atomic instructions by refactoring Clang's CGAtomic.

CGAtomic does not just emit straight LLVM instructions, for various reasons:

1. There is not a single LLVM instruction for some atomic operation
2. Applies type-prunning (shouldCastToInt)
3. While usually AtomicExpandPass is responsible for lowering atomic operations the target does not support, Clang sometimes pre-lowers if the target does not feature native instruction (shouldUseLibcall)
4. For unusual type sizes, atomic operations are also pre-lowered to libcall for performance reasons
5. Select between different algorithms (EmitAtomicUpdateValue, EmitAtomicUpdateLibcall, EmitAtomicUpdateOp)
6. ...

This draft only refactors AtomicInfo into a language-independent parts. EmitAtomicOp lowers more operations wihtout using AtomicInfo but should use common code as well.

Only EmitAtomicLoadOp and EmitAtomicCompareExchange are sketched here. EmitAtomicCompareExchangeLibcall unfortunately reuses Expected/Desired pointers generated by Clang's CodeGen that we cannot reuse for front-end independent code. To make this refactoring NFC though, one could add callbacks to the derived clang::AtomicInfo to generate expected/desired values/pointers on demand.

[Meinersbur]

Became part of llvm#92364