[Objective-C] Fix "weak-unavailable" warning with -fobjc-weak

Summary: clang should produce the same errors Objective-C classes that cannot be assigned to weak pointers under both -fobjc-arc and -fobjc-weak. Check for ObjCWeak along with ObjCAutoRefCount when analyzing pointer conversions. Add an -fobjc-weak pass to the existing arc-unavailable-for-weakref test cases to verify the behavior is the same.

Reviewers: rsmith, doug.gregor, rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31006

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299014 91177308-0d34-0410-b5e6-96231b3b80d8