[dev.typeparams] cmd/compile/internal/types2: use Checker-provided type parameter IDs when possible

This is a port of https://golang.org/cl/317472.

For #46003.

Change-Id: Ie7b8880d43d459527b981ed4f60ee4d80a3cd17a
Reviewed-on: https://go-review.googlesource.com/c/go/+/320149


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>