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`.

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -239,7 +239,7 @@ impl<'a, 'b> DocVisitor for CoverageCalculator<'a, 'b> {
                 let should_be_ignored = i
                     .item_id
                     .as_def_id()
-                    .and_then(|def_id| self.ctx.tcx.parent(def_id))
+                    .and_then(|def_id| self.ctx.tcx.opt_parent(def_id))
                     .and_then(|def_id| self.ctx.tcx.hir().get_if_local(def_id))
                     .map(|node| {
                         matches!(