Split `SolverDelegate` back out from `InferCtxtLike` This is because in order to uplift things like the `Generalizer` and other `TypeRelation`s, we want to be able to interface with `InferCtxtLike` (and `InferCtxt` as its implementation), rather that `SolverDelegate`, which only really exists as a hack to be able to define some downstream methods in `rustc_type_ir`. r? lcnr