Start cutting down `rustc_query_system`
The query system is implemented in `rustc_query_system`, `rustc_middle`, and `rustc_query_impl`. `rustc_query_system` is hamstrung by not having access to `TyCtxt`, and there seems to be consensus to eliminate it. It's contents can be moved into the other two crates. Moving as much stuff as possible to `rustc_query_impl` is preferred, because `rustc_middle` is already so big.
This PR starts this process. It moves one small function to `rustc_middle` and a good chunk of code to `rustc_query_impl`.
Once `rustc_query_system` is gone (or at least shrunk down a lot more) some of the traits like `DepContext`, `QueryContext`, and `QueryDispatcher` will be removable.
r? @Zalathar
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