McALLEN, Tex. — At the Republican debate Wednesday and throughout the campaign, candidates led by Donald J. Trump have assailed illegal immigration, and some have questioned whether children who are born to immigrants in this country illegally should be considered American citizens. But here on the Texas border some communities are engaged in activities that go beyond talk, enforcing some of the toughest rules in the country limiting the types of ID parents can show to receive copies of birth certificates. The result has been a refusal to issue birth certificates to many of the Texas-born children of immigrants here...