[RFC] Make clang builtin math functions `constexpr` with LLVM libc to support C++23 constexpr math functions

[lntue]

First PR attempting to add APFloat::exp for single precision using LLVM libc’s expf : [APFloat] Add exp function for APFloat::IEEESsingle using expf implementation from LLVM libc. by lntue · Pull Request #143959 · llvm/llvm-project · GitHub