Tag build swift-DEVELOPMENT-SNAPSHOT-2016-10-17-a
Merge remote-tracking branch 'origin/swift-3.1-branch' into stable

* origin/swift-3.1-branch:
  Explicitly pass an isysroot to avoid the SDKROOT overriding the deployment target. This fixes the green dragon builders after r284416.
  Improve the CHECK lines in debug-options.c by separating out the check for debug info kind and dwarf version.
  Update testcase for r284416.
  Driver/Darwin: Set the DWARF version based on the deployment target.