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

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lntue wrote:

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

I'm finishing up MSVC support on the LLVM libc side to address @AaronBallman 's comment above.

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