Evolve this Haunter or wait

Evolve. I would be more concerned with Shadow Ball disappearing than Shadow Claw coming back which is unlikely to happen.

I also vote for evolve. I would LOVE to catch a Haunter (or even a Ghastly) with those stats at that level. I have several Gengars, all around level 20, that I have not been able to justify the dust usage to power up. But to get one ready out of the box would be great.

If Shadow Claw comes back, you can always use a Charge TM and try for it.

Evolve, and give him Hex/Shadow Ball, who knows both Hex and Shadow Ball would become legacy and might be even buffed in the future?

Evolve. Even if Shadow Claw might come back one day, you can always get a TM and switch it to Shadow Claw.