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

[Lucas Mellone via cfe-commits]

lknknm wrote:

> Yeah. I think that was exactly the reason. I think it would be less controversial if we let Clang treat fmax etc. constexpr only since C++23 mode.

Could you elucidate? I'm afraid this change of mine could be non-conforming somehow. In any case, do you think we should update this failing test in the scope of this PR?

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