Rollup merge of #142767 - nnethercote:symbol-cleanups, r=petrochenkov

Some symbol and PathRoot cleanups

I'm looking into unifying how we join and print paths. Here are some preliminary cleanups.

r? ``@petrochenkov``

clippy_lints/src/bool_assert_comparison.rs

@@ -56,7 +56,7 @@ fn is_impl_not_trait_with_bool_out<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -
         .and_then(|trait_id| {
             cx.tcx.associated_items(trait_id).find_by_ident_and_kind(
                 cx.tcx,
-                Ident::from_str("Output"),
+                Ident::with_dummy_span(sym::Output),
                 ty::AssocTag::Type,
                 trait_id,
             )