[dev.typealias] go/types: improved Object printing

- added internal isAlias predicated and test
- use it for improved Object printing
- when printing a basic type object, don't repeat type name
  (i.e., print "type int" rather than "type int int")
- added another test to testdata/decls4.src

For #18130.

Change-Id: Ice9517c0065a2cc465c6d12f87cd27c01ef801e6
Reviewed-on: https://go-review.googlesource.com/35093


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>