Remove `tls::with_related_context`. This function gets the current `ImplicitCtxt` and checks that its `tcx` matches the passed-in `tcx`. It's an extra bit of sanity checking: when you already have a `tcx`, and you need access to the non-`tcx` parts of `ImplicitCtxt`, check that your `tcx` matches the one in `ImplicitCtxt`. However, it's only used in two places: `start_query` and `current_query_job`. The non-checked alternatives (`with_context`, `with_context_opt`) are used in more places, including some where a `tcx` is available. And things would have to go catastrophically wrong for the check to fail -- e.g. if we somehow end up with multiple `TyCtxt`s. In my opinion it's just an extra case to understand in `tls.rs` that adds little value. This commit removes it. This avoids the need for `tcx` parameters in a couple of places. The commit also adjusts how `start_query` sets up its `ImplicitCtxt` to more closely match how similar functions do it, i.e. with `..icx.clone()` for the unchanged fields. r? @oli-obk