Hard bifurcate the state into nil receiver and non-nil receiver, so that

we don't need to use the DoneEvaluation hack when check for 
ObjCMessageExpr.

PreVisitObjCMessageExpr() only checks for undefined receiver or arguments.

Add checker interface EvalNilReceiver(). This is a 'once-and-done' interface.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90296 91177308-0d34-0410-b5e6-96231b3b80d8