Mercedes-Benz has developed a tendency to make bold promises with its concept cars. And at the recent North American International Auto Show in Detroit, the German carmaker has done so again with its latest design and technology showpiece, the Concept S-Class Coupe. The car’s exterior lines appear borrowed from the brand’s CLA-Class cars but with more chiseled side panels and an overall sportier feel. The two-door car’s chrome frame surrounds both the front and rear windows, which are free from B-pillars,…