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

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

This is definitely a requirement to implement this, but we also have to make the non-`__builtin_`-prefixed versions `constexpr`, since libc++ doesn't control their definitions. I'm pretty sure there was a PR that does this for some math function, but I can't find it right now.

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