[libc++] Use __enable_if_t and is_integral in cstddef

[ldionne]

Oops, but the CI is failing due to a circular dependency.

[philnik]

• Try to fix CI

libcxx/include/__type_traits/enable_if.h

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___TYPE_TRAITS_ENABLE_IF_H
 #define _LIBCPP___TYPE_TRAITS_ENABLE_IF_H
 
 #include <__config>
-#include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <bool, class _Tp = void> struct _LIBCPP_TEMPLATE_VIS enable_if {};

libcxx/include/cstddef

Types:
     byte // C++17
 
 }  // std
 
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/integral_constant.h>
 #include <__type_traits/is_integral.h>
 #include <stddef.h>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 

 
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 14
 namespace std  // purposefully not versioned
 {
 enum class byte : unsigned char {};
 
-
-template <bool> struct __enable_if_integral_imp {};
-template <> struct __enable_if_integral_imp<true> { using type = byte; };
-template <class _Tp> using _EnableByteOverload = typename __enable_if_integral_imp<__libcpp_is_integral<_Tp>::value>::type;
-
 constexpr byte  operator| (byte  __lhs, byte __rhs) noexcept
 {
     return static_cast<byte>(
       static_cast<unsigned char>(
          static_cast<unsigned int>(__lhs) | static_cast<unsigned int>(__rhs)
     ));
 }
 

 
 constexpr byte  operator~ (byte __b) noexcept
 {
     return static_cast<byte>(
       static_cast<unsigned char>(
         ~static_cast<unsigned int>(__b)
     ));
 }
+
+template <class _Tp>
+using _EnableByteOverload = __enable_if_t<is_integral<_Tp>::value, byte>;
+
 template <class _Integer>
   constexpr _EnableByteOverload<_Integer> &
   operator<<=(byte& __lhs, _Integer __shift) noexcept
   { return __lhs = __lhs << __shift; }
 
 template <class _Integer>
   constexpr _EnableByteOverload<_Integer>
   operator<< (byte  __lhs, _Integer __shift) noexcept