[libc++] Remove _LIBCPP_CONSTEVAL

[ldionne]

Summary

It was only used in one place, and it seems entirely valid to use
constexpr unconditionally in that location.

Note that a different change was attempted, i.e. using consteval
unconditionally. However, this led to http://llvm.org/PR60709.

libcxx/.clang-format

AttributeMacros: ['_LIBCPP_HIDE_FROM_ABI',
                   '_LIBCPP_OVERRIDABLE_FUNC_VIS',
                   '_LIBCPP_EXCEPTION_ABI',
                   '_LIBCPP_ENUM_VIS',
                   '_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS',
                   '_LIBCPP_INTERNAL_LINKAGE',
                   '_LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION',
                   '_LIBCPP_HIDE_FROM_ABI_AFTER_V1',
                   '_LIBCPP_INLINE_VISIBILITY',
-                  '_LIBCPP_CONSTEVAL',
                   '_LIBCPP_NOALIAS',
                   '_LIBCPP_USING_IF_EXISTS',
                   '_LIBCPP_DEPRECATED',
                   '_LIBCPP_DEPRECATED_IN_CXX11',
                   '_LIBCPP_DEPRECATED_IN_CXX14',
                   '_LIBCPP_DEPRECATED_IN_CXX17',
                   '_LIBCPP_DEPRECATED_IN_CXX20',
                   '_LIBCPP_NODISCARD',

libcxx/include/__compare/ordering.h

 class partial_ordering;
 class weak_ordering;
 class strong_ordering;
 
 template<class _Tp, class... _Args>
 inline constexpr bool __one_of_v = (is_same_v<_Tp, _Args> || ...);
 
 struct _CmpUnspecifiedParam {
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEVAL
+  _LIBCPP_HIDE_FROM_ABI constexpr
   _CmpUnspecifiedParam(int _CmpUnspecifiedParam::*) noexcept {}
 
   template<class _Tp, class = enable_if_t<!__one_of_v<_Tp, int, partial_ordering, weak_ordering, strong_ordering>>>
   _CmpUnspecifiedParam(_Tp) = delete;
 };
 
 class partial_ordering {
   using _ValueT = signed char;

libcxx/include/__config

 #  if __has_attribute(__enable_if__)
 #    define _LIBCPP_PREFERRED_OVERLOAD __attribute__((__enable_if__(true, "")))
 #  endif
 
 #  ifndef __SIZEOF_INT128__
 #    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
 #    define _LIBCPP_NOALIAS
 #  endif
 
 #  if __has_attribute(__using_if_exists__)
 #    define _LIBCPP_USING_IF_EXISTS __attribute__((__using_if_exists__))