[analyzer] [RetainCountChecker] [NFC] Remove SummaryLog

The complicated machinery for passing the summary log around is actually
only used for one thing! To figure out whether the "dealloc" message was
sent.

Since I have tried to extend it for other uses and failed (it's actually
very hard to use), I think it's much better to simply use a tag and
remove the summary log altogether.

Differential Revision: https://reviews.llvm.org/D56228

llvm-svn: 350864