Keep type-op region constraints in borrowck

Under `-Zassumptions-on-binders` the next solver stores its region
constraints in the `InferCtxt` instead of registering region
obligations. The canonical type-op path never copied them into
`QueryResponse`, so they were dropped once the query's inference
context went away. Borrowck then saw a type op with no constraints and
lost the outlives error entirely.

`QueryRegionConstraints` now carries the constraint next to the old
style constraints and assumptions. It stays unspanned while passing
through a canonical query and the caller attaches its own origin span
when consuming the response.

Borrowck accumulates these in `MirTypeckRegionConstraints` through
`ConstraintConversion`, the same way it handles everything else, and
destructures them into NLL outlives constraints at the end of MIR type
checking. Implied bound normalization was dropping the same constraint
before lexical regionck, so that path registers it now as well.