The chairman of Cherry Creek Mortgage Co. Inc., Colorado's largest locally-based residential mortgage firm, has resigned. Wil Armstrong announced his resignation late Friday, a day after the Greenwood Village-based company said it was changing its previous policy of denying health-insurance benefits to same-sex couples in the wake of a lawsuit. In his brief announcement, Armstrong said he had resigned Thursday as chairman and a member of the board of directors, "effective immediately." “I am…