cmd/compile/internal/types: change NewNamed to use TypeObject

Semantically, TypeObject is the interface that go/types.TypeName would
implement, so we might as well use that instead of the more generic
Object (analog to go/types.Object) for NewNamed. In practice, this
doesn't really matter though, because we use *ir.Name for almost all
Objects anyway.

Also, remove VarObject: its last use was removed in CL 302071 (March
2021).

Change-Id: I6e987ecd419105e09f74fc8d60cadb61bcbc751f
Reviewed-on: https://go-review.googlesource.com/c/go/+/345810


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