[Objective-c] Fix a crash in WeakObjectProfileTy::getBaseInfo.

The crash occurs in WeakObjectProfileTy::getBaseInfo when getBase() is
called on an ObjCPropertyRefExpr object whose receiver is an interface.
This commit fixes the crash by checking the type of the receiver and
setting IsExact to true if it is an interface.

rdar://problem/25208167

Differential Revision: http://reviews.llvm.org/D18268

llvm-svn: 263818