Make fewer types generic over QueryContext
While trying to refactor `rustc_query_system::query::QueryContext` to make it dyn-safe, I noticed some smaller things:
* QueryConfig doesn't need to be generic over QueryContext
* ~~The `kind` field on QueryJobId is unused~~
* Some unnecessary where clauses
* Many types in `job.rs` where generic over `QueryContext` but only needed `QueryContext::Query`.
If handle_cycle_error() could be refactored to not take `error: CycleError<CTX::Query>`, all those bounds could be removed as well.
Changing `find_cycle_in_stack()` in job.rs to not take a `tcx` argument is the only functional change here. Everything else is just updating type signatures. (aka compile-error driven development ^^)
~~Currently there is a weird bug where memory usage suddenly skyrockets when running UI tests. I'll investigate that tomorrow.
A perf run probably won't make sense before that is fixed.~~
EDIT: `kind` actually is used by `Eq`, and re-adding it fixed the memory issue.
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