go/types, types2: ensure that Alias.actual is set in NewAlias

go/types, types2: ensure that Alias.actual is set in NewAlias

Types returned by the go/types API must be immutable (or at least
concurrency safe), but NewAlias returned an alias without actual set.

Ensure that actual is set by unaliasing. Also make some superficial
simplifications to unalias, and avoid indirection where unnecessary.

Fixes golang/go#65455

Change-Id: Ic9a020da5accf9032056a924b65c9e9e08cb2e0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/560915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@google.com>