A taxpayer group said today that Massachusetts cities and towns have saved $80 million on health care since the state enacted a controversial law last year that forced teachers, fire fighters, and other municipal employees to give up some of their collective bargaining rights. The Massachusetts Taxpayers Foundation, a business-funded watchdog that advocated for the new law, said in its report that the overhaul was likely to save more than the $100 million annually that had initially been projected by the group.