Remove the `_description_fns` module.

`rustc_queries` generates a macro and two modules. One of the modules
looks like this:
```
mod _description_fns {
    ...

    #[allow(unused_variables)]
    pub fn hir_module_items<'tcx>(tcx: TyCtxt<'tcx>, key: LocalModDefId) -> String {
	format!("getting HIR module items in `{}`", tcx.def_path_str(key))
    }

    ...
}
```
Members of this module are then used in `TaggedQueryKey::description`.

This commit removes the `_description_fns` module entirely. For each
query we now instead generate a description closure that is used
instead. This closure is passed in the modifiers list.

This change simplifies `rustc_queries` quite a bit. It requires adding
another query modifier, but query modifiers are how other query-specific
details are already passed to the declarative macros, so it's more
consistent.