Hooked up the necessary machinery to allow the retain/release checker reference

back to the summary used when evaluating the statement associated with a
simulation node. This is now being used to help improve the checker's
diagnostics. To get things started, the checker now emits a path diagnostic
indicating that 'autorelease' is a no-op in GC mode.

Some of these changes are exposing further grossness in the interface between
BugReporter and the ExplodedGraph::Trim facilities. These really need to be
cleaned up one day.

llvm-svn: 64881