[dev.regabi] go/types: convert untyped arguments to delete

This is a port of CL 285059 to go/types. The error assertion is updated
to match go/types error for assignment, which has been improved.

Change-Id: Icdd2751edea0abef7c84feadcbf9265d71239ade
Reviewed-on: https://go-review.googlesource.com/c/go/+/289716


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