[libcxx] [test] Mark tests for libcxx specific implementation details with LIBCPP_ONLY()

[mstorsjo]

Summary

This matches an existing marking in enum.path.format.pass.cpp.

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

 int main(int, char**) {
   typedef fs::copy_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 short>::value, ""); // Implementation detail
+  LIBCPP_ONLY(static_assert(std::is_same<UT, unsigned short>::value, "")); // Implementation detail
 
   typedef check_bitmask_type<E, E::skip_existing, E::update_existing> BitmaskTester;
   assert(BitmaskTester::check());
 
   static_assert(
           E::none == ME(0),
         "Expected enumeration values do not match");
   // Option group for copy_file

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

 
 int main(int, char**) {
   typedef fs::directory_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, "");
+  LIBCPP_ONLY(static_assert(std::is_same<UT, unsigned char>::value, ""));
 
   typedef check_bitmask_type<E, E::follow_directory_symlink, E::skip_permission_denied> BitmaskTester;
   assert(BitmaskTester::check());
 
   static_assert(
         E::none                     == ME(0) &&
         E::follow_directory_symlink == ME(1) &&
         E::skip_permission_denied   == ME(2),
         "Expected enumeration values do not match");
 
 
   return 0;
 }

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

 int main(int, char**) {
   typedef fs::file_type 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, signed char>::value, ""); // Implementation detail
+  LIBCPP_ONLY(static_assert(std::is_same<UT, signed char>::value, "")); // Implementation detail
 
   static_assert(
           E::none == ME(0) &&
           E::not_found == ME(-1) &&
           E::regular == ME(1) &&
           E::directory == ME(2) &&
           E::symlink == ME(3) &&
           E::block == ME(4) &&
           E::character == ME(5) &&
           E::fifo == ME(6) &&
           E::socket == ME(7) &&
           E::unknown == ME(8),
         "Expected enumeration values do not match");
 
   return 0;
 }

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

 int main(int, char**) {
   typedef fs::perms 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 >::value, ""); // Implementation detail
+  LIBCPP_ONLY(static_assert(std::is_same<UT, unsigned >::value, "")); // Implementation detail
 
   typedef check_bitmask_type<E, E::group_all, E::owner_all> BitmaskTester;
   assert(BitmaskTester::check());
 
   static_assert(
         E::none         == ME(0) &&
 
         E::owner_read   == ME(0400) &&