Auto merge of #121462 - compiler-errors:eq-and-sub, r=lcnr

Combine `Sub` and `Equate`

Combine `Sub` and `Equate` into a new relation called `TypeRelating` (that name sounds familiar...)

Tracks the difference between `Sub` and `Equate` via `ambient_variance: ty::Variance` much like the `NllTypeRelating` relation, but implemented slightly jankier because it's a more general purpose relation.

r? lcnr

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -904,7 +904,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             &ObligationCause::dummy(),
             param_env,
             hidden_ty,
-            true,
             &mut obligations,
         )?;
         self.add_goals(GoalSource::Misc, obligations.into_iter().map(|o| o.into()));