[libc++] Revert "Use GCC type traits builtins for remove_cv and remove_cvref (llvm#81386)"

This reverts commit 5535716.
This is only being reverted from the LLVM 19 branch as a
convenience to avoid breaking some IDEs which were not ready
for that change.

Fixes llvm#99464

Author: ldionne

libcxx/include/__type_traits/remove_cv.h

@@ -10,25 +10,32 @@
 #define _LIBCPP___TYPE_TRAITS_REMOVE_CV_H
 
 #include <__config>
+#include <__type_traits/remove_const.h>
+#include <__type_traits/remove_volatile.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if __has_builtin(__remove_cv) && !defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
 struct remove_cv {
   using type _LIBCPP_NODEBUG = __remove_cv(_Tp);
 };
 
-#if defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
-using __remove_cv_t = typename remove_cv<_Tp>::type;
+using __remove_cv_t = __remove_cv(_Tp);
 #else
 template <class _Tp>
-using __remove_cv_t = __remove_cv(_Tp);
-#endif
+struct _LIBCPP_TEMPLATE_VIS remove_cv {
+  typedef __remove_volatile_t<__remove_const_t<_Tp> > type;
+};
+
+template <class _Tp>
+using __remove_cv_t = __remove_volatile_t<__remove_const_t<_Tp> >;
+#endif // __has_builtin(__remove_cv)
 
 #if _LIBCPP_STD_VER >= 14
 template <class _Tp>

libcxx/include/__type_traits/remove_cvref.h

@@ -20,26 +20,21 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if defined(_LIBCPP_COMPILER_GCC)
+#if __has_builtin(__remove_cvref) && !defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
-struct __remove_cvref_gcc {
-  using type = __remove_cvref(_Tp);
-};
-
-template <class _Tp>
-using __remove_cvref_t _LIBCPP_NODEBUG = typename __remove_cvref_gcc<_Tp>::type;
+using __remove_cvref_t _LIBCPP_NODEBUG = __remove_cvref(_Tp);
 #else
 template <class _Tp>
-using __remove_cvref_t _LIBCPP_NODEBUG = __remove_cvref(_Tp);
+using __remove_cvref_t _LIBCPP_NODEBUG = __remove_cv_t<__libcpp_remove_reference_t<_Tp> >;
 #endif // __has_builtin(__remove_cvref)
 
 template <class _Tp, class _Up>
-using __is_same_uncvref = _IsSame<__remove_cvref_t<_Tp>, __remove_cvref_t<_Up> >;
+struct __is_same_uncvref : _IsSame<__remove_cvref_t<_Tp>, __remove_cvref_t<_Up> > {};
 
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp>
 struct remove_cvref {
-  using type _LIBCPP_NODEBUG = __remove_cvref(_Tp);
+  using type _LIBCPP_NODEBUG = __remove_cvref_t<_Tp>;
 };
 
 template <class _Tp>