[Bug libstdc++/97132] assume_aligned is not constexpr

[fsb4000 at yandex dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97132

--- Comment #4 from fsb4000 at yandex dot ru ---
(In reply to Jonathan Wakely from comment #3)
> I'll test this:
> 
>      {
>        static_assert(std::has_single_bit(_Align));
>        _GLIBCXX_DEBUG_ASSERT((std::uintptr_t)__ptr % _Align == 0);
> -      return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
> +      if (std::is_constant_evaluated())
> +       return __ptr;
> +      else
> +       return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
>      }
>  #endif // C++2a

Yes, should be fine. 
We do same for Microsoft STL.