Rollup merge of #137314 - lcnr:cycles-with-unknown-kind, r=compiler-errors

change definitely unproductive cycles to error

builds on top of #136824 by adding a third variant to `PathKind` for paths which may change to be coinductive in the future but must not be so right now. Most notably, impl where-clauses of not yet coinductive traits.

With this, we can change cycles which are definitely unproductive to a proper error. This fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/114. This does not affect stable as we keep these cycles as ambiguous during coherence.

r? ````````@compiler-errors```````` ````````@nikomatsakis````````

compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs

@@ -440,7 +440,7 @@ impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
             match (child_mode, nested_goal.source()) {
                 (
                     ChildMode::Trait(_) | ChildMode::Host(_),
-                    GoalSource::Misc | GoalSource::NormalizeGoal(_),
+                    GoalSource::Misc | GoalSource::TypeRelating | GoalSource::NormalizeGoal(_),
                 ) => {
                     continue;
                 }