go/types, types2: factor out hasDots to check for ... arguments in calls (cleanup)

go/types, types2: factor out hasDots to check for ... arguments in calls (cleanup)

This further reduces the differences between go/types and types2.

Change-Id: Ie651c13dd12ecf043b8be92655d48d1ce32d4c5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/562777
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>