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_metadata/src/rmeta/decoder.rs

@@ -991,7 +991,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             _ => false,
         };
 
-        ModChild { ident, res, vis, span, macro_rules }
+        ModChild { ident, res, vis, span, macro_rules, reexport_chain: Default::default() }
     }
 
     /// Iterates over all named children of the given module,