[libc] [llvm] [APFloat] Add exp function for APFloat::IEEESsingle using expf implementation from LLVM libc. (PR #143959)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

@Ariel-Burton, fyi. I think the scope of this PR is safe, but it's a question where things are going.

https://github.com/llvm/llvm-project/pull/143959