* Fix external pointer propagation in nested dereferences

and fix the count of indirections for addrof of member dereferences
(&A->b).

In nested dereferences, a dereference of an external pointer may give
a new external pointer.  For example, if A is an external pointer,
then *A and A->b should also be considered as external pointers when
appropriate (e.g., in **A or *(A->b)).

In addition, a member dereference is a dereference, so we need to
count it when counting the number of indirections in ProbeChecker.
If we don't, *(&A->b) won't be rewritten correctly as &A->b will be
considered a pointer to an external pointer.

* Tests for the count of indirections in nested dereferences