Objective-C. Prevents a crash generating AST for a
a property assignment due to numerous user errors.
Cannot come up with a reasonable test case due to
array of user errors before the crash point.
rdar://17813651.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217825 91177308-0d34-0410-b5e6-96231b3b80d8