rustc: Panic by default in DefIdTree::parent

Only crate root def-ids don't have a parent, and in majority of cases the argument of `DefIdTree::parent` cannot be a crate root.
So we now panic by default in `parent` and introduce a new non-panicing function `opt_parent` for cases where the argument can be a crate root.

Same applies to `local_parent`/`opt_local_parent`.

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

@@ -1906,7 +1906,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         GeneratorKind::Async(AsyncGeneratorKind::Fn) => self
                             .tcx
                             .parent(generator_did)
-                            .and_then(|parent_did| parent_did.as_local())
+                            .as_local()
                             .map(|parent_did| hir.local_def_id_to_hir_id(parent_did))
                             .and_then(|parent_hir_id| hir.opt_name(parent_hir_id))
                             .map(|name| {