Pull implied bound computation out of borrowck

[tiif]

Open this draft PR so it's easier to discuss, feel free to point out any logic / soundness / error handling issues :>

Current status:

• The implementation is mostly done, currently trying to get core to build and resolve ICEs found in test suite
• The test for normalizing opaques while computing implied bounds trait-system-refactor-initiative#159 successfully error in the new solver
• need more clean ups
• need to add test for super trait bounds can result in unconstrained regions #136547

[tiif]

This change in this file is mainly duplicating the existing function to not use nll var. The new implied bound query needs these functions, but it doesn't makes sense to use nll var there. boxy gave some ideas on how to deduplicate this, but I haven't got to it yet.