cmd/compile/internal/types2: no need to validate substituted instances

When substituting a type instance, we rely on the instance being
expanded and do not call validType, so there is need to depend on
subster.pos for error reporting or to use subst.check for creating the
new Named type. Errors will be reported for the unsubstituted instance.

This is a superficial change, but justifies some later simplification
where we don't have access to pos or check.

Change-Id: I1f3f12aa245d821512c6242ad829c940f20afae4
Reviewed-on: https://go-review.googlesource.com/c/go/+/342150


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