A bill protecting historical monuments  passed the state Senate Thursday.   The vote was 24-7.   It followed more than two hours of debate,  in which opponents questioned whether the measure was  necessary.    But the  sponsor --  Sen. Gerald Allen (R-Tuscaloosa) -- says history is under assault from the forces of political correctness.    Under the bill,  local governments are blocked from making changes to monuments that have stood for more than 20 years, and may not rename certain streets and buildings.   Younger monuments could be moved with the permission of a new entity, the Committee on Alabama Monument Protection.   The bill now heads to the House.