go/types: ensure that we always get a new signature in expandNamed

CL 349412 introduced a bug when Checker.subst does not return a new
signature: we were still setting the receiver to the instantiated type.

I'm not sure how this could manifest in practice (other than confusing
object strings). It's possible that I could generate a testdata-driven
test for this, but in the interest of time I just added a test to verify
object strings.

Change-Id: I29bc8e1419ddc4574755c3def52d18cb71c738eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/350143


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>