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_passes/src/liveness.rs

@@ -332,9 +332,9 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
         let def_id = local_def_id.to_def_id();
 
         // Don't run unused pass for #[derive()]
-        if let Some(parent) = self.tcx.parent(def_id)
-            && let DefKind::Impl = self.tcx.def_kind(parent.expect_local())
-            && self.tcx.has_attr(parent, sym::automatically_derived)
+        let parent = self.tcx.local_parent(local_def_id);
+        if let DefKind::Impl = self.tcx.def_kind(parent)
+            && self.tcx.has_attr(parent.to_def_id(), sym::automatically_derived)
         {
             return;
         }