go/types, types2: use a map instead of a field for marking in validType

With this change validType doesn't modify global state anymore.
It also eliminates the need for an extra field in each object.

Preparation for fixing issue #48962.

Change-Id: If241ec77ff48911d5b43d89adabfb8ef54452c6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/378176


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>