cmd/compile/internal/types2: disallow type cycles through type parameter lists

cmd/compile/internal/types2: disallow type cycles through type parameter lists

If we reach a generic type that is part of a cycle
and we are in a type parameter list, we have a cycle
through a type parameter list, which is invalid.

Fixes #49439.

Change-Id: Ia6cf97e1748ca0c0e61c02841202050091365b0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/361922
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>