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 aaef3b8.

libcxx/.clang-format

@@ -37,6 +37,7 @@ AttributeMacros: ['_LIBCPP_HIDE_FROM_ABI',
  '_LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION',
  '_LIBCPP_HIDE_FROM_ABI_AFTER_V1',
  '_LIBCPP_INLINE_VISIBILITY',
+ '_LIBCPP_CONSTEVAL',
  '_LIBCPP_NOALIAS',
  '_LIBCPP_USING_IF_EXISTS',
  '_LIBCPP_DEPRECATED',

libcxx/include/__compare/ordering.h

@@ -40,7 +40,7 @@ template<class _Tp, class... _Args>
 inline constexpr bool __one_of_v = (is_same_v<_Tp, _Args> || ...);
 
 struct _CmpUnspecifiedParam {
- _LIBCPP_HIDE_FROM_ABI consteval
+ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEVAL
  _CmpUnspecifiedParam(int _CmpUnspecifiedParam::*) noexcept {}
 
  template<class _Tp, class = enable_if_t<!__one_of_v<_Tp, int, partial_ordering, weak_ordering, strong_ordering>>>

libcxx/include/__config

@@ -686,6 +686,12 @@ _LIBCPP_BEGIN_NAMESPACE_STD _LIBCPP_END_NAMESPACE_STD
 # define _LIBCPP_HAS_NO_INT128
 # endif
 
+# ifndef __cpp_consteval
+# define _LIBCPP_CONSTEVAL _LIBCPP_CONSTEXPR
+# else
+# define _LIBCPP_CONSTEVAL consteval
+# endif
+
 # if __has_attribute(__malloc__)
 # define _LIBCPP_NOALIAS __attribute__((__malloc__))
 # else