Fix stack overflow in exhaustiveness due to recursive HIR opaque hidden types
This fixes several spicy non-trivial recursive opaque definitions inferred from HIR typeck, ensuring that they don't cause stack overflows in exhaustiveness code, which currently reveals opaques manually in a way that is not overflow aware (as opposed to something like the normalizer folders).
These should eventually be outright rejected, but today (some) non-trivial recursive opaque definitions are accepted, and changing that requires an FCP, so for now just make sure we don't stack overflow :^)
Fixes https://github.com/rust-lang/rust/issues/139402
r? lcnr
9ea9339435