[clang][dataflow] Model assignment to derived class from base. (#85064)

This is a relatively rare case, but

- It's still nice to get this right,
- We can remove the special case for this in
`VisitCXXOperatorCallExpr()` (that
  simply bails out), and
- With this in place, I can avoid having to add a similar special case
in an
  upcoming patch.