title: Accidentally modified an iOS SDK header file, now I cannot compile in simulator
body: <p>I don't know how I modified a iOS SDK file, but Xcode say I did.  Here is what they reported.</p>
  
  <pre><code>fatal error: file '/Applications/Xcode 2.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator7.0.sdk/System/Library/Frameworks/UIKit.framework/Headers/UIFontDescriptor.h' has been modified since the precompiled header '/Users/m/Library/Developer/Xcode/DerivedData/ModuleCache/24CDWDK5BTYJE/UIKit.pcm' was built
  note: after modifying system headers, please delete the module cache at '/Users/m/Library/Developer/Xcode/DerivedData/ModuleCache/24CDWDK5BTYJE'
  1 error generated.
  </code></pre>
  
  <p>I tried to look for this DerivedData but could not. I also went into organizer to delete it and did a clean, even after that, no luck.</p>
  
  <p>How do I restore this UIFontDescriptor.h to its original form?</p>