[clang] [clang] Constant evaluation support for `__builtin_fmax{,f,l}` and `__builtin_fmin{,f,l}` (PR #225824)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

The current PR title seems incorrect to me.

It should be non-controversial to make the `__builtin` versions `constexpr`, even unconditionally (as they're extensions). However, isn't this already done ([Godbolt link](https://godbolt.org/z/qT3KsvsvP))?

https://github.com/llvm/llvm-project/pull/225824