Rollup merge of #156048 - JonathanBrouwer:diverging_ty_vids, r=WaffleLapkin

Make `diverging_type_vars` a vec of `TyVid`

r? @lcnr

The following changes, in separate commits:
* Make its elements a `TyVid`, since there should never by any other types than `TyVars` in there
* Make it a vec, since it being a set doesn't make much sense. You never really should do a `contains` on it, since you should normalize the tyvids in the set to their root var first.

Author: JonathanBrouwer