go/types, types2: use asNamed(t) instead of t.(*Named) type assertions

go/types, types2: use asNamed(t) instead of t.(*Named) type assertions

Preparation for the introduction of alias types.
Because asNamed is not exported, existing external
tests continue to use t.(*Named).

Change-Id: I4754b406dd6b23030d3703a486d6f6620b2464fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/522876
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>