Improve pretty-printing of HirIdValidator errors

This now uses `node_to_string` for both missing and seen Ids, which includes
the snippet of code for which the Id was allocated.
Also removes the duplicated printing of `HirId`, as `node_to_string` includes that already.
Similarly, changes all other users of `node_to_string` that do so, and changes the output of `node_to_string`, which is now "$hirid ($what `$span` in $path)".

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -133,7 +133,7 @@ impl<'tcx, 'a> GeneratorData<'tcx, 'a> {
                                         .cloned()
                                         .unwrap_or_else(|| {
                                             bug!(
-                                                "node_type: no type for node `{}`",
+                                                "node_type: no type for node {}",
                                                 ty::tls::with(|tcx| tcx
                                                     .hir()
                                                     .node_to_string(await_expr.hir_id))