[dev.typeparams] go/types: re-use existing code for Interface.Complete

This is a port of CL 321751 to go/types, adjusted to use token.Pos, and
to exclude a missing position from a panic message (an unresolved
comment on the original CL).

Change-Id: I5814067aecb67aca9d73f2093fb6004b769924f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/324756


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