Tag build swift-DEVELOPMENT-SNAPSHOT-2017-01-28-a
Merge remote-tracking branch 'origin/swift-4.0-branch' into stable

* origin/swift-4.0-branch:
  ARM-Darwin: re-enable -momit-leaf-frame-pointer.
  Fix linkage of static locals in available_externally functions to be DiscardableODR/linkonce_odr
  [ubsan] Sanity-check shift amounts before truncation (fixes PR27271)