Or, maybe more accurately, a visitor from a parallel present day.
Here’s their inherently spoilery description of what I’ll happily bet you is a post-credits teaser scene:
It takes place in an airport… involves Wolverine….and Professor X. The new scene has the two of them walking into the airport and then going into a freeze frame.
So, not only is Xavier back from the dead, he’s walking again. Sounds like some timey wimey Days of Future Past fun to me.