Don't mark associated items as traits

This caused the following false positive:

```
warning: unresolved link to `Default::default`
 --> /home/joshua/rustc2/default.rs:1:14
  |
1 | /// Link to [Default::default()]
  |              ^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(broken_intra_doc_links)]` on by default
note: this item resolved to a trait, which did not match the disambiguator 'fn'
 --> /home/joshua/rustc2/default.rs:1:14
  |
1 | /// Link to [Default::default()]
  |              ^^^^^^^^^^^^^^^^^^
```

Author: jyn514

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -415,7 +415,8 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
                                 AnchorFailure::Method
                             }))
                         } else {
-                            Ok((ty_res, Some(format!("{}.{}", kind, item_name))))
+                            let res = Res::Def(item.kind.as_def_kind(), item.def_id);
+                            Ok((res, Some(format!("{}.{}", kind, item_name))))
                         }
                     } else {
                         self.variant_field(path_str, current_item, module_id)

src/test/rustdoc/intra-link-trait-item.rs

@@ -0,0 +1,3 @@
+#![deny(broken_intra_doc_links_)]
+/// Link to [Default::default()]
+pub fn f() {}