[dev.typeparams] cmd/compile/internal/types2: "comparable" must not be visible before Go 1.18

While at it, clean up the setup of comparable in universe.go.

Fixes #46090

Change-Id: I9655b3e137a03763d677d9a2a730c5570ccff6dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/331517


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>