go/types, types2: factor out shared for-range checking code into range.go

go/types, types2: factor out shared for-range checking code into range.go

For go/types, generate its range.go file from the corresponding types2 file.

Change-Id: Iaff3ecbf1c536143c92f7b50e2461140469f9280
Reviewed-on: https://go-review.googlesource.com/c/go/+/655536
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>