Revert "[libc++] Remove _LIBCPP_CONSTEVAL"

It causes mysterious memory leaks when comparing std::string, see GitHub
Issue #60709 and the code review.

> All supported compilers support `consteval`, so there is no more need for the macro.
>
> Reviewed By: ldionne, Mordante, #libc
>
> Spies: libcxx-commits
>
> Differential Revision: https://reviews.llvm.org/D143489

This reverts commit aaef3b82.