Tag build swift-DEVELOPMENT-SNAPSHOT-2016-12-07-a
Merge remote-tracking branch 'origin/swift-3.1-branch' into stable

* origin/swift-3.1-branch:
  [ARM] Fix for 64-bit CAS expansion on ARM32 with -O0
  AArch64: fix 128-bit cmpxchg at -O0 (again, again).