go/types, types2: better error position for invalid (infinite) types

go/types, types2: better error position for invalid (infinite) types

Provide an explicit start position to Checker.cycleError for better
control over the reported error.

For #65711.

Change-Id: Ie3016523442d75f348a033c1b944db493943f433
Reviewed-on: https://go-review.googlesource.com/c/go/+/567916
Auto-Submit: 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>
Reviewed-by: Robert Griesemer <gri@google.com>