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

[Lucas Mellone via cfe-commits]

lknknm wrote:

> The current PR title seems incorrect to me.
Why? Should it be `fmax{,f,l}` etc?

> 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))?

I don't know, considering the discussion inside the `constexpr copysign` PR. I also don't think it is already done. Take a look at https://github.com/llvm/llvm-project/pull/222953/changes. 

Moreover, can you check the failing tests here? I think `constexpr-cxx23-clang.pass.cpp` is failing here on linux because I added `BIfmax` etc as constexpr.


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