Add "checker caching" to GRExprEngine::CheckerVisit to progressively build

a winowed list of checkers that actually do something for a given StmtClass.
As the number of checkers grows, this may potentially significantly reduce
the number of checkers called at any one time.  My own measurements show that
for the ~20 registered Checker objects, only ~5 of them respond at any one time
to a give statement.  While this isn't a net performance win right now (there
is a minor slowdown on sqlite.3) this improvement does greatly improve debugging
when stepping through the checkers used to evaluate a given statement.

llvm-svn: 106884