SIMI VALLEY, Calif. — Along with the back-and-forth bickering over Donald Trump, the Republican presidential contest took a substantive and serious turn in Wednesday’s prime-time debate, with candidates wrangling over immigration, gay marriage and foreign affairs. The policy shift quieted Trump, the brash billionaire who has roiled the GOP field, for long stretches during the three-hour debate and appeared to come as a relief to other candidates who have struggled to break through. Carly Fiorina, the only woman in the GOP field, was one of the main benefactors, launching an emotional plea for defunding Planned Parenthood, touting her experience in business and taking aim at Trump for derogatory comments he made about her appearance.