Do not keep track of the set of visited Objective-C containers when

performing the search for overridden methods. We very rarely see the
same container twice, and in those rare cases we still have the
fallback of the second SmallPtrSet to eliminate duplicates. Good for
~1.5% -fsyntax-only speedup on the code in <rdar://problem/11004361>.

llvm-svn: 156103