Cambridge-based Ironwood Pharmaceuticals Inc.’s drug to treat irritable bowel syndrome with constipation has won marketing approval in Europe. Linaclotide, for adults, will be launched in Europe next year as Constella. The drug received US approval in August. Ironwood will sell the drug in the United States with a marketing partner, Forest Laboratories, under the brand name Linzess. A Ladenburg Thalmann analyst estimated Constella could bring in revenue of $189 million in 2018.