[libcxx] [test] Add LIBCPP_ONLY() around another test for an implementation detail

libcxx/test/std/input.output/filesystems/fs.enum/enum.perm_options.pass.cpp

 int main(int, char**) {
   typedef fs::perm_options E;
   static_assert(std::is_enum<E>::value, "");
 
   // Check that E is a scoped enum by checking for conversions.
   typedef std::underlying_type<E>::type UT;
   static_assert(!std::is_convertible<E, UT>::value, "");
 
-  static_assert(std::is_same<UT, unsigned char >::value, ""); // Implementation detail
+  LIBCPP_ONLY(static_assert(std::is_same<UT, unsigned char >::value, "")); // Implementation detail
 
   typedef check_bitmask_type<E, E::replace, E::nofollow> BitmaskTester;
   assert(BitmaskTester::check());
 
   static_assert(
         E::replace  == ME(1) &&
         E::add      == ME(2) &&
         E::remove   == ME(4) &&
         E::nofollow == ME(8),
         "Expected enumeration values do not match");
 
   return 0;
 }