Auto merge of #100705 - compiler-errors:issue-100620, r=oli-obk

Avoid reporting overflow in `is_impossible_method`

Fixes #100620

We're evaluating a new predicate in a different param-env than it was checked during typeck, so be more careful about handling overflow errors. Instead of using `FulfillmentCtxt`, using `InferCtxt::evaluate_obligation` by itself will give us back the overflow error, so we can throw it away properly.

This may give us more false-positives, but it doesn't regress the `<HashMap as Iterator>::rev` example that originally motivated adding `is_impossible_method` in the first place.