[analyzer] Use a FoldingSet to cache simple RetainSummary instances, rather...

[analyzer] Use a FoldingSet to cache simple RetainSummary instances, rather than explicitly keeping DoNothing and StopTracking summaries and nothing else.

I tried to test the effects of this change on memory usage and run time, but what I saw on retain-release.m was indistinguishable from noise (debug and release builds). Even so, some caveman profiling showed 101 cache hits that we would have generated new summaries for before (i.e. not default or stop summaries), and the more code we analyze, the more memory we should save.

Maybe we should have a standard project for benchmarking the retain count checker's memory and time?

llvm-svn: 153007