go/types: better error message for invalid untyped nil conversion

This is port of CL 354049 for types2 to go/type.

The change is identical, but for some tweaks to the error message/position
in tests, since when go/types reports the exact operation "cannot convert"
instead of the general "invalid operation" like types2.

Updates #48784

Change-Id: I3e99f2721501d23187fd0a8970eb1de28e0c41d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/354050


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>