go/types: refer to structural rather than single underlying type in errors

go/types: refer to structural rather than single underlying type in errors

This is a port of CL 362997 from types2 to go/types. Some error
positions were adjusted in tests.

Change-Id: I6a932aee1a8d9bcbf4cd8c16a95bbb41b5c7e13f
Reviewed-on: https://go-review.googlesource.com/c/go/+/363990
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>