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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> @jcranmer-intel @hubert-reinterpretcast l, I have applied the changes you requested.

Thanks @Serosh-commits. For info, I am hoping that we could land https://github.com/llvm/llvm-project/pull/199808 first to avoid issues with parallel changes to overlapping code. Also, I'll be on vacation for a few days both next week and the week after.

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