go/types, types2: in SetUnderlying, set Named.fromRHS if not set yet

This is necessary for cycle detection over imported types whose
underlying types are set by importers with SetUnderlying.

Preparation for fixing issue #48962.

Change-Id: I3218cda7feb06440fdb8345c94bcaa5f7d64e94e
Reviewed-on: https://go-review.googlesource.com/c/go/+/379694


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>