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 rust-lang/trait-system-refactor-initiative#114. This does not affect stable as we keep these cycles as ambiguous during coherence.

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

Author: matthiaskrgr