go/types: generate version.go from types2-equivalent

No semantic changes. Generated code is identical, and the
generator is not run as part of the type checker proper.

Safe for 1.27.

Change-Id: I92821b5f4de2baeaf5c43e78c300d3f62905d28e
Reviewed-on: https://go-review.googlesource.com/c/go/+/785003
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>