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

* origin/swift-3.1-branch:
  [analyzer][scan-build-py] subprocess output handling reviewed in clang module
  [www] Add nullability questions to analyzer FAQ.
  [www] Add analyzer FAQ about not releasing ivars in -dealloc.
  [analyzer] Update alpha and potential checker documentation, esp. alpha.valist