[libc++][NFC] Simplify enable_if for std::copy optimization

[ldionne]

Summary

Get rid of the is_trivially_copy_assignable_unwrapped helper, which
is only used in one place, and use iter_value_type instead of
iterator_traits<T>::value_type.

[Mordante]

LGTM when the CI is happy.

libcxx/include/__algorithm/copy.h

#endif
      )
     return std::__copy_impl<_InValueT*, _InValueT*, _OutValueT*>(__first, __last, __result);
   const size_t __n = static_cast<size_t>(__last - __first);
   if (__n > 0)
     ::__builtin_memmove(__result, __first, __n * sizeof(_OutValueT));
   return std::make_pair(__first + __n, __result + __n);
 }
 
-template <class _Iter>
-using __is_trivially_copy_assignable_unwrapped =
-    _And<__is_cpp17_contiguous_iterator<_Iter>, is_trivially_copy_assignable<__iter_value_type<_Iter> > >;
-
-template <class _InIter,
-          class _OutIter,
-          class = __enable_if_t<is_same<typename remove_const<typename iterator_traits<_InIter>::value_type>::type,
-                                        typename iterator_traits<_OutIter>::value_type>::value
-                             && __is_trivially_copy_assignable_unwrapped<_InIter>::value
-                             && __is_trivially_copy_assignable_unwrapped<_OutIter>::value> >
+template <class _InIter, class _OutIter,
+          __enable_if_t<is_same<typename remove_const<__iter_value_type<_InIter> >::type, __iter_value_type<_OutIter> >::value
+                      && __is_cpp17_contiguous_iterator<_InIter>::value
+                      && __is_cpp17_contiguous_iterator<_OutIter>::value
+                      && is_trivially_copy_assignable<__iter_value_type<_OutIter> >::value, int> = 0>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX11
 pair<reverse_iterator<_InIter>, reverse_iterator<_OutIter> >
 __copy_impl(reverse_iterator<_InIter> __first,
             reverse_iterator<_InIter> __last,
             reverse_iterator<_OutIter> __result) {
   auto __first_base = std::__unwrap_iter(__first.base());
   auto __last_base = std::__unwrap_iter(__last.base());
   auto __result_base = std::__unwrap_iter(__result.base());