Who works well against Diana and LeBlanc?

It's just as easy for Diana to one shot Riven with a single combo... There's no play advantage there. The best way to keep a snowbally assassin--like Diana or LeBlanc--down is to keep their early game farm to a minimum. Safe farmers with high poke damage are most effective; Varus, Corki, Twisted Fate, and Anivia should all perform reasonably well against either of them. Syndra and Cho'Gath from above are good options.

What you're really looking for here is the ability to disrupt or prevent their combo. Poke them out, and try your best to lay down hard cc during their combo. Diana in particular loses her engage if she misses Q and if you stay out of melee range. She has no disengage options, so if you can hard cc her after she goes in or as she's trying to go in you can get favorable trades while minimizing damage.

LeBlanc's trades come in short bursts that you should aim to interrupt. It's easier to catch a LeBlanc by aiming your disruptions at her Distortion points because they're fixed return points and most players will move in and out without delay. If you can force her to use Distortion to dodge you can usually force a favorable trade, as well.

Hope that helps.