Add support for all variants of `ast::WherePredicate` in clean/mod.rs

[jroesch]

This is related to #20041 and #20002. We currently only support the cleaning of the BoundPredicate variant, we should support RegionPredicate, and once implemented EqualityPredicate as well.

[tomjakubowski]

I'll get on this once #20002 lands, or if you prefer and think the types are relatively stable I can send you a patch to add to #20002 (it should be relatively small).

[jroesch]

I made some progress on #20049 earlier today, might want to wait until that lands since it changes the types slightly. I think #20002 should be good to go as soon as Niko gives the final okay.