Fix computation of parent types of a classinfo.
Fix computation of parent types of a classinfo.

The previously used `rebase` did not rewrire outer this references.
The right thing to do here is an asSeenFrom.

sjrd/dotty,som-snytt/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,som-snytt/dotty,som-snytt/dotty,dotty-staging/dotty,lampepfl/dotty