Use `DECLARE_SPECIAL_CONSTANTS` macro here.

You can add the signed versions to `DECLARE_SPECIAL_CONSTANTS` macro and use it here: https://github.com/llvm/llvm-project/blob/main/libc/test/UnitTest/FPMatcher.h#L92

Use the second template parameter for the `sign`, and use `enable_if_t` on the return type instead. Also mark this function `constexpr` if possible.