go/types, types2: use errorCause instead of reportf in comparableType

go/types, types2: use errorCause instead of reportf in comparableType

If the error cause is not further specified (empty string),
avoid allocating a new errorCause. This makes using errorCauses
as boolean signals efficient.

While at it, fix an error message for incomparable arrays:
report the array type rather than its underlying type.

Change-Id: I844b18a76695330ca726932ee760aa89635f6a38
Reviewed-on: https://go-review.googlesource.com/c/go/+/654575
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>