resolve: Preserve reexport chains in ModChildren

This may be potentially useful for
- avoiding uses of `hir::ItemKind::Use`
- preserving documentation comments on all reexports
- preserving and checking stability/deprecation info on reexports
- all kinds of diagnostics

compiler/rustc_middle/src/arena.rs

@@ -119,6 +119,7 @@ macro_rules! arena_types {
             [] external_constraints: rustc_middle::traits::solve::ExternalConstraintsData<'tcx>,
             [decode] doc_link_resolutions: rustc_hir::def::DocLinkResMap,
             [] closure_kind_origin: (rustc_span::Span, rustc_middle::hir::place::Place<'tcx>),
+            [] mod_child: rustc_middle::metadata::ModChild,
         ]);
     )
 }