[clang] [Clang] Add constexpr eval for cmath builtins (PR #194327)

[via cfe-commits]

Serosh-commits wrote:

@hubert-reinterpretcast thanks for your detailed review it really helped me learn a lot and understand the real issues behind the changes while  addressing your comments, I have also done some additional cleanups that I felt were necessary If anything still looks off or needs improvement let me know 

https://github.com/llvm/llvm-project/pull/194327