Allow Trait inheritance with cycles on associated types take 2
This reverts the revert of #79209 and fixes the ICEs that's occasioned by that PR exposing some problems that are addressed in #80648 and #79811.
For easier review I'd say, check only the last commit, the first one is just a revert of the revert of #79209 which was already approved.
This also could be considered part or the actual fix of #79560 but I guess for that to be closed and fixed completely we would need to land #80648 and #79811 too.
r? `@nikomatsakis`
cc `@Aaron1011`
44e526b2c3