Remove references to 'Checker' and 'GRTransferFuncs' from

GRStateManager.  Having these references was an abstraction violation,
as they really should only be known about GRExprEngine.

This change required adding a new 'ProcessAssume' callback in
GRSubEngine.  GRExprEngine implements this callback by calling
'EvalAssume' on all registered Checker objects as well as the
registered GRTransferFunc object.

llvm-svn: 92549