rustdoc: Fix ICE with cross-crate default impls

This adds a special code path for impls which are listed as default impls to
ensure that they're loaded correctly.

src/librustdoc/doctree.rs

@@ -202,6 +202,8 @@ pub struct DefaultImpl {
     pub unsafety: ast::Unsafety,
     pub trait_: ast::TraitRef,
     pub id: ast::NodeId,
+    pub attrs: Vec<ast::Attribute>,
+    pub whence: Span,
 }
 
 pub struct Macro {