[libc++] Remove workaround for broken __is_trivially_copyable on old GCC

[ldionne]

Summary

All supported versions of GCC now do the right thing.

[• Quuxplusone]

If buildkite is happy, I'm happy! (I still think there's a chance this will break someone, so I'm glad it's being done as a separate commit; thanks.)

libcxx/include/type_traits

 template <class _Tp>
 _LIBCPP_INLINE_VAR _LIBCPP_CONSTEXPR bool is_standard_layout_v
     = is_standard_layout<_Tp>::value;
 #endif
 
 // is_trivially_copyable;
 
 template <class _Tp> struct _LIBCPP_TEMPLATE_VIS is_trivially_copyable
-#if __has_feature(is_trivially_copyable)
     : public integral_constant<bool, __is_trivially_copyable(_Tp)>
-#elif _GNUC_VER >= 501
-    : public integral_constant<bool, !is_volatile<_Tp>::value && __is_trivially_copyable(_Tp)>
-#else
-    : integral_constant<bool, is_scalar<typename remove_all_extents<_Tp>::type>::value>
-#endif
     {};
 
 #if _LIBCPP_STD_VER > 14 && !defined(_LIBCPP_HAS_NO_VARIABLE_TEMPLATES)
 template <class _Tp>
 _LIBCPP_INLINE_VAR _LIBCPP_CONSTEXPR bool is_trivially_copyable_v
     = is_trivially_copyable<_Tp>::value;
 #endif
 

libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_trivially_copyable.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // type_traits
 
 // is_trivially_copyable
 
-// GCC has not implemented Core 2094 which makes volatile
-// qualified types trivially copyable.
-// XFAIL: gcc
-
 #include <type_traits>
 #include <cassert>
 #include "test_macros.h"
 
 template <class T>
 void test_is_trivially_copyable()
 {
     static_assert( std::is_trivially_copyable<T>::value, "");