[analyzer] Fix crash with pointer to members values

This fix unifies all of the different ways we handled pointer to
members into one.  The crash was caused by the fact that the type
of pointer-to-member values was `void *`, and while this works
for the vast majority of cases it breaks when we actually need
to explain the path for the report.

rdar://problem/64202361

Differential Revision: https://reviews.llvm.org/D85817