Cleanups and fixes to the nil-receiver checker, some of it fallout the

initial transition of the nil-receiver checker to the Checker
interface as done in r89745.  Some important changes include:

1) We consolidate the BugType object used for nil receiver bug
reports, and don't include the type of the returned value in the
BugType (which would be wrong if a nil receiver bug was reported more
than once)

2) Added a new (temporary) flag to CheckerContext: DoneEvauating.
This is used by GRExprEngine when evaluating message expressions to
not continue evaluating the message expression if this flag is set.
This flag is currently set by the nil receiver checker.  This is an
intermediate solution to allow the nil-receiver checker to properly
work as a plug-in outside of GRExprEngine.  Basically, this flag
indicates that the entire message expression has been evaluated, not
just a precondition (which is what the nil-receiver checker does).
This flag *should not* be repurposed for general use, but just to pull
more things out of GRExprEngine that already in there as we devise a
better interface in the Checker class.

3) Cleaned up the logic in the nil-receiver checker, making the
control-flow a lot easier to read.

llvm-svn: 89804