title: Xcode 10 Beta CompileSwiftSources failed
body: <p>Trying Xcode 10 GM Seed (10A254a) with our project and the build failed with message "command CompileSwiftSources failed with non-zero exit code".</p>
  
  <p>So far I have tried:
  1. Clean and Deep Clean (shift+alt+cmd+k)
  2. Delete the DerivedData folder
  3. Restart Xcode
  4. Restart laptop</p>
  
  <p>all failed to address this issue.</p>