preserve the original visibility for the "list stem" node

Without this, the `vis` does not wind up in the tree anywhere, and
then we get ICEs because the node-ids it refers to are not present.
The motivation seemed to be documentation, but `ListStem` HIR nodes
are ignored in rustdoc, from what I can tell.

Author: nikomatsakis

src/librustc/hir/lowering.rs

@@ -3139,12 +3139,8 @@ impl<'a> LoweringContext<'a> {
                     });
                 }
 
-                // Privatize the degenerate import base, used only to check
-                // the stability of `use a::{};`, to avoid it showing up as
-                // a re-export by accident when `pub`, e.g. in documentation.
                 let def = self.expect_full_def_from_use(id).next().unwrap_or(Def::Err);
                 let path = P(self.lower_path_extra(def, &prefix, ParamMode::Explicit, None));
-                *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
             }
         }