Remove the CFGElement "Invalid" state.

Use Optional<CFG*> where invalid states were needed previously. In the one case
where that's not possible (beginAutomaticObjDtorsInsert) just use a dummy
CFGAutomaticObjDtor.

Thanks for the help from Jordan Rose & discussion/feedback from Ted Kremenek
and Doug Gregor.

Post commit code review feedback on r175796 by Ted Kremenek.