[libc++] Fix incorrect REQUIRES on a locale-dependent test

[ldionne]

Summary

The test doesn't depend specifically on the en_US.UTF-8 locale, instead
it depends on whether localization support exists, period.

[Mordante]

LGTM!

libcxx/test/libcxx/iterators/iterator.requirements/iterator.assoc.types/iterator.traits/locale_dependent.compile.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
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-has-no-localization
 // UNSUPPORTED: libcpp-no-concepts
 
-// REQUIRES: locale.en_US.UTF-8
-
 #include <iterator>
 
 #include <istream>
 #include <ostream>
 
 static_assert(std::__iterator_traits_detail::__cpp17_iterator<std::istream_iterator<int, std::istream> >);
 static_assert(std::__iterator_traits_detail::__cpp17_iterator<std::istreambuf_iterator<int, std::istream> >);
 static_assert(std::__iterator_traits_detail::__cpp17_iterator<std::ostream_iterator<int, std::ostream> >);