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

[via llvm-commits]

lntue wrote:

> I'm also concerned about having a function declaration but not definition if LLVM_INTEGRATE_LIBC is off.

I wrapped the declarations inside `#ifdef LLVM_INTEGRATE_LIBC`.

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