[analyzer] Look for calls along with lvalue nodes in trackNullOrUndefValue.

r176737 fixed bugreporter::trackNullOrUndefValue to find nodes for an lvalue
even if the rvalue node had already been collected. This commit extends that
to call statement nodes as well, so that if a call is contained within
implicit casts we can still track the return value.

No test case because node reclamation is extremely finicky (dependent on
how the AST and CFG are built, and then on our current reclamation rules,
and /then/ on how many nodes were generated by the analyzer core and the
current set of checkers). I consider this a low-risk change, though, and
it will only happen in cases of reclamation when the rvalue node isn't
available.

<rdar://problem/13340764>