types2: move RangeKeyVal func into compilersupport.go

This function only exists in types2.

Change-Id: If0a8b6aaf496142cefdf088bd5656536f4ed101c
Reviewed-on: https://go-review.googlesource.com/c/go/+/604276


Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>