[analyzer] Find better description for tracked symbolic values

When searching for stores and creating corresponding notes, the
analyzer is more specific about the target region of the store
as opposed to the stored value.  While this description was tweaked
for constant and undefined values, it lacked in the most general
case of symbolic values.

This patch tries to find a memory region, where this value is stored,
to use it as a better alias for the value.

rdar://76645710

Differential Revision: https://reviews.llvm.org/D101041