[objc-arc] Teach the ARC optimizer that objc_sync_enter/objc_sync_exit do not modify the ref count of an objc object and additionally are inert for modref purposes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185769 91177308-0d34-0410-b5e6-96231b3b80d8