[dev.typeparams] cmd/compile/internal/types2: report error for invalid type expression

This bug was introduced by the change from go/ast to syntax which
represents pointer types as (unary) operations rather than dedicated
StarExpr nodes. Accordingly, this bug does not exist for go/types.
It's still ok to backport the test.

Fixes #43125.

Change-Id: Ic55d913f8afc92862856e1eb7c2861d07fc56cfb
Reviewed-on: https://go-review.googlesource.com/c/go/+/278013


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