Chicago Weekend; 74 shot, 11 dead:   The first weekend in August has added another 74 shootings to the estimated 1,700 which have occurred in Chicago so far this year.  Madeline Buckley of the Chicago Tribune reports that eleven people died and 63  were wounded in shootings mostly on the South and West sides of the city.   The victim's ages ranged from 11 to 62.  A police official told reporters that most of the violence was by gang members who shoot into summer crowds at night.   Two 17-year-olds, shot in separate incidents, were the youngest homicide victims.  Tribune data indicates that at least 300 people have died in Chicago shootings this year.   Manhattan Institute scholar Heather MacDonald has this take on the reasons for this violence in today's City Journal.