Clang Code Model is slow for completing class

1. Open a source file.
2. In a sensible place, type "class" and hit Ctrl+Space immediately.
   Nothing will happen. You'd need to wait for more than a second before hitting Ctrl+Space to make it correctly trigger a completion pop-up.

Creator should be able to display the completion pop-up immediately. With the built-in code model, this works correctly.

Found by Squish test tst_CSUP02.