Through the coding we know Tyrunt has Sturdy as it's HA and Tyrantrum has Rock Head as it's HA. Game Freak have confirmed that we're getting HA Tyrunt. Do the tiniest bit of research next time.
You just corrected yourself. Tyrunt gets sturdy as it's HA which means rock head is not programmed to be tyrunt's HA... if it's not programmed to be a hidden ability, then chances are, it's not going to evolve with a hidden ability meaning tyrantrum may not be receiving anything new, just it's preevolution. I going by how the game's mechanics are coded for this to work, yeah tyrantrum's HA is rock head, no shit, but when a pokemon has a hidden ability.. it evolves with it's hidden ability (if the evolution has one) which in this case.. tyrunt doesn't have IT'S hidden ability (sturdy) so there is a possibility it's not going to evolve with one.