arc-repeated-use-of-weak should not warn about IBOutlet properties

Revision r211132 was supposed to disable -Warc-repeated-use-of-weak for
Objective-C properties marked with the IBOutlet attribute. Those properties
are supposed to be weak but they are only accessed from the main thread
so there is no risk of asynchronous updates setting them to nil. That
combination makes -Warc-repeated-use-of-weak very noisy. The previous
change only handled one kind of access to weak IBOutlet properties.
Instead of trying to add checks for all the different kinds of property
accesses, this patch removes the previous special case check and adds a
check at the point where the diagnostic is reported. rdar://21366461

llvm-svn: 270665