[Objective-C] Fix "repeated use of weak" warning with -fobjc-weak

Summary: -Warc-repeated-use-of-weak should produce the same warnings with -fobjc-weak as it does with -objc-arc. Also check for ObjCWeak along with ObjCAutoRefCount when recording the use of an evaluated weak variable. Add a -fobjc-weak run to the existing arc-repeated-weak test case and adapt it slightly to work in both modes.

Reviewers: rsmith, doug.gregor, jordan_rose, rjmccall

Reviewed By: rjmccall

Subscribers: arphaman, rjmccall, cfe-commits

Differential Revision: https://reviews.llvm.org/D31005

llvm-svn: 299011