Law firms, including several Chicago-based offices, are the leading industry among American workplaces that foster LGBT-friendly policies and practices, according to a survey released this week.The Human Rights Campaign Foundation released its 15th annual Corporate Equality Index on Monday, in which it graded large employers across the country based on various LGBT-related workplace practices.Among the 517 companies to receive 100-percent ratings were 13 law firms headquartered in Chicago.Those 13 firms are: Baker & …