Typechecking slice literals, array literals, and array literals using
"..." notation all use very similar logic, but tie into the logic for
checking the OCOMPLIT node in slightly different ways.

By refactoring this function out into a separate helper, it makes it
easier to separate slice and array literals, and the subsequent CL
will further separate array literals that do use "..." notation from
those that do not.

Passes toolstash-check.

Change-Id: I4c572e0d9d08bcc86b5c224bd6f9e1c498726c19
Reviewed-on: https://go-review.googlesource.com/c/go/+/197603
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>