[Analyzer] Fix crash in ObjCPropertyChecker on protocol property

Fix a crash in the ObjCPropertyChecker when analyzing a 'copy' property of an
NSMutable* type in a protocol.

rdar://problem/30766684

Differential Revision: https://reviews.llvm.org/D30482

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@296562 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 8acf4ef934c61633e907154eacfcf15baeb0eed2)

Integration into swift-3.1-branch is tracked by rdar://problem/30772776
2 files changed