[libc++] Remove _LIBCPP_CONSTEVAL

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.

Differential Revision: https://reviews.llvm.org/D145700

libcxx/.clang-format

@@ -37,7 +37,6 @@ 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 _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>>>

libcxx/include/__config

@@ -695,12 +695,6 @@ _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