title: ARSessionConfiguration unresolved in Xcode 9 GM
body: <p>I have created an <code>ARKit</code> project using a beta version of Xcode 9, which I was able to run on my real device without issues.<br>
  Yesterday, I upgraded to Xcode 9 GM, and without touching anything, Xcode shows multiple errors, saying it does not know <code>ARSessionConfiguration</code> i.e.:</p>
  
  <blockquote>
    <p>Use of undeclared type 'ARSessionConfiguration'</p>
  </blockquote>
  
  <p>and:</p>
  
  <blockquote>
    <p>Use of undeclared type 'ARWorldTrackingSessionConfiguration'</p>
  </blockquote>
  
  <p>...for this code:</p>
  
  <pre><code>let session = ARSession()
  var sessionConfig: ARSessionConfiguration = ARWorldTrackingSessionConfiguration()
  </code></pre>
  
  <p>I have imported <code>ARKit</code> and am using the <code>ARSCNViewDelegate</code> in my ViewController.<br>
  When opening the project from the beta version of Xcode, it does not show the errors and I can again run the app on my phone.</p>
  
  <p>Any idea how I can fix this?</p>