title: Xcode 9 code completion and highlighting not working
body: <p>I have seen many threads about this online, but not for <code>Xcode 9</code>. For some reason code completion and syntax highlighting is no longer working. Last week it was fine.</p>
  
  <p>Using the GM build.</p>
  
  <p>Anyone have this issue or know how to resolve it? A restart of Xcode and my computer did not help.</p>