[dev.typeparams] go/types: accept embedded interface elements

This is a port of CL 321689 to go/types. It differs from that CL in the
uses of the position, AST and error APIs, and in not factoring out an
unimplemented() helper (this helper didn't already exist in go/types, so
it seemed cleaner to defer adding it).

Change-Id: I577a57297caf35eb7a23f63f3f52037a7bb528ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/326069


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