In case there was any doubt that the drug price backlash is having little effect on the willingness of big pharma to pay top dollar for promising new drugs, the $624 million buyout of a Canadian biotech by Marlborough-based Sunovion Pharmaceuticals ought to make that fact clear. Sunovion, a privately held subsidiary of Japan-based Sumitomo Dainippon Pharma Co., announced late Wednesday that it plans to buy Toronto-based Cynapsus Therapeutics Inc.