We usually point people to the Jelly Escalation service for this question, but it's not available in Cloud and it's going away in version 7 of JIRA (not sure how Atlassian are going to replace it either).
If you were on the server version of Jira, I'd also start talking about doing it with the Script Runner (or your own "service"), or other plugins from the marketplace. But I don't think there's anything available for Cloud.
On a technical note, all the solutions work outside the workflow really - they need the workflow to have a transition of "resolved -> Closed", and they automate the "run a filter and push each resulting issue through that transition" process, running as a regular service.
But I have a feeling that your best option for Cloud would be to perform that service remotely - write a script or program that runs on your own systems and uses Cloud's REST interface to do the filter and transition trick.
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