[dev.regabi] go/types: no "declared but not used" errors for invalid var decls

This is a port of CL 274615, adapted to go/types. The only change was in
the positioning of expected errors in vardecl.src: in go/types they are
positioned on the identifier.

Change-Id: Iab03265a7c4287749373e4380c6db6a95f262f30
Reviewed-on: https://go-review.googlesource.com/c/go/+/289712


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>