Merge remote-tracking branch 'origin/swift-3.1-branch' into stable

* origin/swift-3.1-branch:
  [API Notes] Look for framework API notes in Headers/PrivateHeaders.
  [API Notes] Add a command-line option for Swift version selection.
  [Modules] Fix module hash computation when module file extensions are involved.
  [API Notes] Add Swift versioning to API notes files