holding that "consideration of race or ethnicity by the [University of Texas] law school for the purposes of achieving a diverse student body is not a compelling interest under the Fourteenth Amendment."

finding that the state's interest in diversity at a law school was not compelling and declaring that school's admissions policy, which employed racial classifications, unconstitutional

dismissing state law claims that arguably constituted unfair labor practice because deciding those claims would have required determination of whether state law relied upon was consistent with substantive provisions of federal law and fashioning of remedy, both of which are exclusive responsibilities of National Labor Relations Board