What you are looking for is a device commonly known as a "doorway."
When you go into the main room of the bar, look across the room. See that rectangular opening in the wall?
After you shoot the two grans (and the barkeep expresses his horror at the loss of life), walk through the opening. Turn left. follow the corridor (technically, this is
Be careful when you get around the end; there's an unarmed brawler around the corner, and a sniper up on the ledge.