title: Can't use safe areas in Xcode 9 after upgrading minimum deployment target to iOS 9
body: <p>I have just changed my app from supporting iOS 8 and up to supporting iOS 9 and up.  I believe I've done this properly, since when I now build my app, I'm getting warnings for everything in the app which was deprecated in iOS 9.  </p>
  
  <p>I am using Xcode 9, GM seed.</p>
  
  <p>The problem is that I cannot enable the "Use Safe Area Layout Guides" toggle in any of my storyboards.  When I do, I get a warning "Safe Area Layout Guide before iOS 9.0".  Is there some additional setting that I need to update?  A clean and build did not fix the problem.</p>