Lazily resolve type-alias-impl-trait defining uses

by using an opaque type obligation to bubble up comparisons between opaque types and other types

Also uses proper obligation causes so that the body id works, because out of some reason nll uses body ids for logic instead of just diagnostics.

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -853,6 +853,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 | ty::PredicateKind::Subtype(..)
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::Coerce(..)
+                | ty::PredicateKind::OpaqueType(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
             };
         }