Resolve intra-doc links in summary desc

Before:

![rustdoc-intra-doc-link-summary-before](https://user-images.githubusercontent.com/1593513/122623069-9d995e80-d04f-11eb-8d46-ec2ec126bb5e.png)

After:

![rustdoc-intra-doc-link-summary](https://user-images.githubusercontent.com/1593513/122623076-a4c06c80-d04f-11eb-967a-f5916871c34b.png)

src/librustdoc/formats/cache.rs

@@ -292,13 +292,14 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
                     // which should not be indexed. The crate-item itself is
                     // inserted later on when serializing the search-index.
                     if item.def_id.index().map_or(false, |idx| idx != CRATE_DEF_INDEX) {
+                        let desc = item.doc_value().map_or_else(String::new, |x| {
+                            short_markdown_summary(&x.as_str(), &item.link_names(&self.cache))
+                        });
                         self.cache.search_index.push(IndexItem {
                             ty: item.type_(),
                             name: s.to_string(),
                             path: path.join("::"),
-                            desc: item
-                                .doc_value()
-                                .map_or_else(String::new, |x| short_markdown_summary(&x.as_str())),
+                            desc,
                             parent,
                             parent_idx: None,
                             search_type: get_index_search_type(&item, &self.empty_cache, self.tcx),