[clang] [llvm] Add emitAtomicCompareExchangeBuiltin helper function (PR #101966)

[Michael Kruse via cfe-commits]

Meinersbur wrote:

`TargetLibraryInfoImpl` is not contained in the backends, but implemented entirely in [`TargetLibraryInfo.cpp`](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Analysis/TargetLibraryInfo.cpp). 

`TargetLowering` is implemented in the backend, but is also optional. If not provided, the max size of atomic operations is assumed to be 16. I think that frontends having to resolve the ABI information independently would be worse.

https://github.com/llvm/llvm-project/pull/101966