Objective-C ARC. Do not warn about properties with both

IBOutlet and weak attributes when accessed being
unpredictably set to nil because usage of such properties
are always single threaded and its ivar cannot be set
to nil asynchronously. // rdar://15885642 


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211132 91177308-0d34-0410-b5e6-96231b3b80d8