[analyzer] Fix skipping the call during inlined defensive check suppression.

When bugreporter::trackExpressionValue() is invoked on a DeclRefExpr,
it tries to do most of its computations over the node in which
this DeclRefExpr is computed, rather than on the error node (or whatever node
is stuffed into it). One reason why we can't simply use the error node is
that the binding to that variable might have already disappeared from the state
by the time the bug is found.

In case of the inlined defensive checks visitor, the DeclRefExpr node
is in fact sometimes too *early*: the call in which the inlined defensive check
has happened might have not been entered yet.

Change the visitor to be fine with tracking dead symbols (which it is totally
capable of - the collapse point for the symbol is still well-defined), and fire
it up directly on the error node. Keep using "LVState" to find out which value
should we be tracking, so that there weren't any problems with accidentally
loading an ill-formed value from a dead variable.

Differential Revision: https://reviews.llvm.org/D67932