ptr_refs exposed a problem in ClangASTContext's implementation: it
uses an accessor to downcast a QualType to an
ObjCObjectPointerType, but the accessor is not fully general.
getAs() is the safer way to go.
I've added a test case that uses ptr_refs in a way that would
crash before the fix.
<rdar://problem/31363513>
llvm-svn: 303110
Sean Callanan