[clang] [clang] constexpr built-in fma function. (PR #113020)

[Michael Jones via cfe-commits]

michaelrj-google wrote:

I've created an issue for planning out the LLVM-libc + clang code sharing (https://github.com/llvm/llvm-project/issues/114109). That being said I think FMA might be fine to implement within clang, due to its simplicity. Additionally, LLVM-libc may call `__builtin_fma`: https://github.com/llvm/llvm-project/blob/main/libc/src/__support/FPUtil/FMA.h#L32

CC: @lntue 

https://github.com/llvm/llvm-project/pull/113020