[analyzer] Convert RetainCountChecker to use CallEvent as much as possible.

This ended allowing quite a bit of cleanup, and some minor changes.

- CallEvent makes it easy to use hasNonZeroCallbackArg more aggressively, which
  we check in order to avoid false positives with callbacks that might release
  the object.
- In order to support this for functions which consume their arguments, there
  are two new ArgEffects: DecRefAndStopTracking and DecRefMsgAndStopTracking.
  These act just like StopTracking, except that if the object only had a
  return count of +1 it's now considered released instead (so we still get
  use-after-free messages).
- On the plus side, we no longer have to special-case
  +[NSObject performSelector:withObject:afterDelay:] and friends.
- The use of IdentifierInfos in the method summary cache is now hidden; only
  the ObjCInterfaceDecl gets passed around most of the time.
- Since we cache all "simple" summaries and check every function call, there is
  no real benefit to having NULL stand in for default summaries anymore.
- Whitespace, unused methods, etc.

Even more simplification to come when we get check::postCall and can unify all
these other post* checks.

llvm-svn: 159555