title: Command swiftc crashes on Xcode10
body: <p>I need to do some non-trivial refactoring in a mixed objc &amp; swift project.</p>
  
  <p>But after some code editing, the new Xcode10 compiler crashes every time.</p>
  
  <p>I'm sure there are some errors in my refactored code, but the compiler is not able to show where. It just says :</p>
  
  <pre><code>Command /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc failed with exit code 1
  </code></pre>
  
  <p>So frustrating! Impossible to debug!</p>
  
  <p>Of course, I updated the whole project to Swift 4.2, so I cannot roll back to Xcode 9.4 :(</p>
  
  <p>Is this a bug? Is there some compilation parameters to make swiftc giving better errors?</p>
  
  <p>Thanks</p>
  
  <p>EDIT: just tried with Xcode 10.1 (beta), and the same problem persists.</p>
  
  <p>EDIT2: after investigations, it seems the problem occurs when I import the objc generated interface header file (the one generally finishing with *-Swift.h). Still no solution.</p>