[dev.typeparams] cmd/compile/internal/types2: type alias decl requires go1.9

Add respective check to type checker.
Remove respective check from the compiler's new type2-based noder.

Updates #31793.

Change-Id: I907e3acab4c136027a8c3db1e9bac301d209c2e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/289570


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