go/types: match types2 error msg formatting for conversion errors

go/types: match types2 error msg formatting for conversion errors

This resolves a TODO. Also, it allows go/types and types2 to use
identical test files in testdata/spec, a prerequisition for sharing
them eventually.

For #54511.

Change-Id: Ia9d11b15ada237c18b6ce8f72732f70134a1a06f
Reviewed-on: https://go-review.googlesource.com/c/go/+/424537
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>