[clang] [libc] [llvm] [clang] Make __builtin_exp and __builtin_expf constexpr. (PR #199808)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

@hubert-reinterpretcast @jcranmer-intel I'd like to understand the next steps for this PR; getting this initial work unblocked will help us start making bigger inroads into the constexpr cmath efforts, so I'd like to keep momentum rolling here if we can.

https://github.com/llvm/llvm-project/pull/199808