Change the checker callback cache in GRExprEngine to be more compact (and IMHO...

Change the checker callback cache in GRExprEngine to be more compact (and IMHO a little easier to understand), and add the same sort of caching for EvalAssume (tied for least-used callback), mostly as proof-of-concept.

Before we go further with these, we should figure out a way to reuse the visit-and-cache code in CheckerVisit.

llvm-svn: 110191