rustdoc: avoid many Symbol to String conversions.

Particularly when constructing file paths and fully qualified paths.
This avoids a lot of allocations, speeding things up on almost all
examples.

compiler/rustc_span/src/symbol.rs

@@ -567,6 +567,7 @@ symbols! {
         doc_spotlight,
         doctest,
         document_private_items,
+        dotdot: "..",
         dotdot_in_tuple_patterns,
         dotdoteq_in_patterns,
         dreg,