gcimporters: allow reusing empty interfaces on the RHS of type decls

We guard against caching or reusing interfaces on the RHS of a type
declaration, because for such interfaces the base type is used as the
interface method receiver type. However, we don't need to do this for
empty interfaces. By refining our guard, we can allow importing the
predeclared 'any' type on the RHS of a type declaration.

Update tests to add more coverage for importing generic export data.
Some accomodation had to be made for the unified builder, which does not
yet fully support generics in export data.

Fixes #49888

Change-Id: I51f329de464fc7309f95991b839ab55868c2924f
Reviewed-on: https://go-review.googlesource.com/c/go/+/367851


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>