Merge remote-tracking branch 'origin/swift-3.0-branch' into stable

* origin/swift-3.0-branch:
  [test/objcmt] Add a follow-up test case for r275600.
  [objcmt] Fix a buffer overflow crash than can occur while modernizing enums.
  [AST] Keep track of the left brace source location of a tag decl.
  [objcmt] Don't add an #import of Foundation unnecessarily, if the NS_ENUM macro is already defined.