[Objective-C] Miscellaneous -fobjc-weak Fixes

Summary: After examining the remaining uses of LangOptions.ObjCAutoRefCount, found a some additional places to also check for ObjCWeak not covered by previous test cases. Added a test file to verify all the code paths that were changed.

Reviewers: rsmith, doug.gregor, rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31007

llvm-svn: 299015