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

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

Differential Revision: https://reviews.llvm.org/D89534

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

@@ -30,7 +30,7 @@ int main(int, char**) {
   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());

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

@@ -30,7 +30,7 @@ int main(int, char**) {
   // 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());

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

@@ -29,7 +29,7 @@ int main(int, char**) {
   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) &&

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

@@ -31,7 +31,7 @@ int main(int, char**) {
   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());