To make sure we are on the same page, we are taling aout links shortened through external *services* such as goo.gl, non just links shortened through markdown that reveal the orginal on mouse-over.

Flagging as "Other" with the description explaining it's a shortened link is good; However if it was a up-voted comment that indicates that it was useful at one time, so it may be good to re-post the original link, If the Original user is no longer available . 

Also commenting to the user that posted it is a good idea, and is probably the best route with new comments.