title: Ignore .metal files when building for iOS Simulator target
body: <p>Whenever I build a project that includes a metal shader to an x86_64 target (iOS simulator), I get a dependency analysis warning:</p>
  
  <p><code>warning: no rule to process file '[File Path]/Shaders.metal' of type sourcecode.metal for architecture x86_64</code></p>
  
  <p>I know this isn't a huge issue but I like to keep my projects free from warnings when I build, so that when a real issue does arise, I actually notice the yellow warning triangle.</p>
  
  <p>Any quick way to get Xcode to ignore metal files for simulator targets?</p>