Remove RefCell around `module_trait_cache`
This builds on https://github.com/rust-lang/rust/pull/82018 and should not be merged before.
## Don't require a `DocContext` for `report_diagnostic`
This is needed for the next commit, which needs mutable access to the `cx` from
within the `decorate` closure.
- Change `as_local_hir_id` to an associated function, since it only
needs a `TyCtxt`
- Change `source_span_for_markdown_range` to only take a `TyCtxt`
## Remove RefCell around module_trait_cache
This is mostly just changing lots of functions from `&DocContext` to `&mut DocContext`.