go/types, types2: pass *TypeName instead of *Named as def (defined) type

go/types, types2: pass *TypeName instead of *Named as def (defined) type

This permits using the same mechanism not just for *Named types but
also future Alias types.

Change-Id: Ia8dee413d8dff7b08ab073ce03a4779603c4f396
Reviewed-on: https://go-review.googlesource.com/c/go/+/524897
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>