[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:

> 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.

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.

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