[analyzer] trackNullOrUndefValue: track last store to non-variables.

When reporting certain kinds of analyzer warnings, we use the
bugreporter::trackNullOrUndefValue mechanism, which is part of public checker
API, to understand where a zero, null-pointer, or garbage value came from,
which would highlight important events with respect to that value in the
diagnostic path notes, and help us suppress various false positives that result
from values appearing from particular sources.

Previously, we've lost track of the value when it was written into a memory
region that is not a plain variable. Now try to resume tracking in this
situation by finding where the last write to this region has occured.

Differential revision: https://reviews.llvm.org/D41253

llvm-svn: 321130