[libcxx] Explicitly return the expected error code in create_directories if the parent isn't a directory

[mstorsjo]

Summary

On windows, going ahead and actually trying to create the directory doesn't return an error code that maps to std::errc::not_a_directory in this case.

This fixes two cases of

TEST_CHECK(ErrorIs(ec, std::errc::not_a_directory))

in filesystems/fs.op.funcs/fs.op.create_directories/create_directories.pass.cpp for windows (in testcases added in 59c72a70121567f7aee347e96b4ac8f3cfe9f4b2).

[mstorsjo]

(For this one, I'd like to have @curdeius opinion, as he was working on this aspect in D93026 and D92769.)

[curdeius]

libcxx/src/filesystem/operations.cpp

1026–1027

is_directory can take a file_status (so parent_st) to avoid requerying the filesystem.

[mstorsjo]

libcxx/src/filesystem/operations.cpp

1026–1027

Good catch, thanks!

[mstorsjo]

Check the existing parent_st instead of re-stat'ing the directory.

[mstorsjo]

Restart CI

[mstorsjo]

@curdeius Does the new version look ok to you?

[curdeius]

Yes, LGTM! Sorry for having forgotten this review.

libcxx/src/filesystem/operations.cpp

     const file_status parent_st = status(parent, m_ec);
     if (not status_known(parent_st))
       return err.report(m_ec);
     if (not exists(parent_st)) {
       __create_directories(parent, ec);
       if (ec && *ec) {
         return false;
       }
-    }
+    } else if (not is_directory(parent_st))
+      return err.report(errc::not_a_directory);
   }
   return __create_directory(p, ec);
 }
 
 bool __create_directory(const path& p, error_code* ec) {
   ErrorHandler<bool> err("create_directory", ec, &p);
 
   if (detail::mkdir(p.c_str(), static_cast<int>(perms::all)) == 0)