Add type-checking logic for range over integers and functions,
behind GOEXPERIMENT=range.
For proposal #61405 (but behind a GOEXPERIMENT).
For #61717.
Change-Id: Ibf78cf381798b450dbe05eb922df82af2b009403
Reviewed-on: https://go-review.googlesource.com/c/go/+/510537
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
