go/types, types2: validType argument must be *Named type

Now that we have a separate top-level entry point for validType
we can use the more narrow type *Named (instead of Type) for its
argument.

Preparation for fixing issue #48962.

Change-Id: I93aee4abc87036c6a68323dc970efe8e617a9103
Reviewed-on: https://go-review.googlesource.com/c/go/+/379434


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