Ensure walk_item visits GlobalAsm NodeId

Travis failures indicated the OuterVisitor#visit_item method caused a
panic. The Visitor's inner visitor actually relies on the visitor
visiting every item's NodeId. I forgot to perform that call in the
ItemGlobalAsm match arm, leading to build breakage. The fix is
simple: call visit_id(...) for ItemGlobalAsm

Author: mrhota

src/librustc/hir/intravisit.rs

@@ -474,7 +474,9 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item) {
             visitor.visit_id(item.id);
             walk_list!(visitor, visit_foreign_item, &foreign_module.items);
         }
-        ItemGlobalAsm(_) => {}
+        ItemGlobalAsm(_) => {
+            visitor.visit_id(item.id);
+        }
         ItemTy(ref typ, ref type_parameters) => {
             visitor.visit_id(item.id);
             visitor.visit_ty(typ);