NLL: handle outlives relations, process region obligations in the MIR type checker #45826

Once #45825 lands, the NLL region solver will be integrated with the MIR type-checker and the rest of our inference infrastructure. However, we do not yet handle "verifys", which is the way that we sometimes encode type_outlives relations. Also, the MIR type checker is not currently invoking process_region_obligations, so outlives relations often do not even get processed.

Right now, you can't actually observe failures as a result of this, or at least I couldn't, I think because of various things that are not currently enforced by the MIR type-checker. But I am filing this bug as a placeholder for now. Mentoring instructions to come.