rustdoc: Link associated items in search index to trait

This is related to isssue #22442 and solves it partly.

This solves the links of associated types and constants, so that they
link to the trait page.

src/librustdoc/html/static/main.js

@@ -34,7 +34,8 @@
                      "macro",
                      "primitive",
                      "associatedtype",
-                     "constant"];
+                     "constant",
+                     "associatedconstant"];
 
     $('.js-only').removeClass('js-only');