A car’s deceleration using the brakes (or any other kind of acceleration on a level road) is limited by the friction between the tires and the road: The maximum acceleration possible is g times the coefficient of friction. Typical coefficients of friction are usually somewhere in the vicinity of 1 or less, so you’re not going to get accelerations much greater than 1g.
Without a seat belt, meanwhile, a passenger is going to be held in his seat by friction, as well. If the friction between the passenger’s butt and the seat is greater than that between the road and the tires, he’ll stay put, and if it’s less, he’ll slide forward in his seat. If nothing else stops him, eventually his body will hit the dashboard, and the upper portion of his body will fold around it and hit the windshield. Given that tires are specifically designed for high friction, and butts and car seats aren’t, I’m going to guess that the butt-seat friction is less, maybe about 0.5 . So for a completely limp rag-doll passenger, it would be possible to hit the windshield.
Of course, real passengers aren’t completely limp rag-dolls, either. A real passenger would probably have enough reaction time to get his arms in front of him to brace against the dashboard, and even if he didn’t, his body won’t fold around the dashboard completely effortlessly, either. So for a real passenger, I think the chance of hitting the windshield at all is pretty small, and even if one did, it’d be unlikely to have enough energy left to break through it.