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

They have been added recently to GCC without support for mangling. This
patch uses them in structs and adds aliases to these structs instead of
the builtins directly.

Author: philnik777

libcxx/include/__type_traits/remove_cv.h

@@ -19,22 +19,17 @@
 
 _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 = __remove_cv(_Tp);
+using __remove_cv_t = typename remove_cv<_Tp>::type;
 #else
 template <class _Tp>
-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> >;
+using __remove_cv_t = __remove_cv(_Tp);
 #endif // __has_builtin(__remove_cv)
 
 #if _LIBCPP_STD_VER >= 14

libcxx/include/__type_traits/remove_cvref.h

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