[dev.typeparams] cmd/compile/internal/types2: resolve decl cycle the same way as in go/types

Minor adjustment to match go/types more closely.

Change-Id: Id79c51f0ecd8cda0f5b68f6e961500f7f22f7115
Reviewed-on: https://go-review.googlesource.com/c/go/+/294270


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