go/types, types2: ensure that signature type bounds are interfaces

Do this by running verification for instantiated signatures
later, after the delayed type parameter set-up had a chance
to wrap type bounds in implicit interfaces where needed.

Fixes #50450

Change-Id: If3ff7dc0be6af14af854830bfddb81112ac575cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/375737


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