Added several guards in transfer functions for "InvalidValues".

Fixed bug in RemoveDeadBindings by implementing a simple "mark-and-sweep"
cleaner over the bindings, starting from the Decls and block-level expressions
that are considered "live" by the Liveness analysis.

Fixed bug in isa<> implementation for class LValue.

Added "VisitDeclRefExpr" to GRConstants so that we explicitly bind the current
value of variable to the Block-level Expression (i.e., when the DeclRefExpr is
at the CFGBlock level).

llvm-svn: 46839