go/types, types2: uniformly use "cannot convert X to type T"

go/types, types2: uniformly use "cannot convert X to type T"

This matches current compiler behavior.

For #55326.

Change-Id: I660bd15f13a8d9eb00fafa937f8261e664b0e065
Reviewed-on: https://go-review.googlesource.com/c/go/+/433276
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>