instantiate query response nested obligations

[lcnr]

While instantiating a query response we may end up emitting nested obligations.

This is annoying and we have to deal with them somehow. We currently put them into the fulfillment context of the caller, but that may not be great for diagnostics. I think we could alternatively loop tem in a separate context while instantiating and OR their certainty with the main goal

[compiler-errors]

For coherence: we already do this in evaluate_predicate so it's not unsound at least :^).

[lcnr]

with rust-lang/rust#119106 instantiating query responses does not result in nested obligations anymore. Closing