Fix handling of item names for HIR

- Handle variants, fields, macros in `Node::ident()`
- Handle the crate root in `opt_item_name`
- Factor out `item_name_from_def_id` to reduce duplication
- Look at HIR before the DefId for `opt_item_name`

  This gives accurate spans, which are not available from serialized
  metadata.

- Don't panic on the crate root in `opt_item_name`
- Add comments

compiler/rustc_hir/src/hir.rs

@@ -2677,6 +2677,9 @@ impl<'hir> Node<'hir> {
             Node::TraitItem(TraitItem { ident, .. })
             | Node::ImplItem(ImplItem { ident, .. })
             | Node::ForeignItem(ForeignItem { ident, .. })
+            | Node::Field(StructField { ident, .. })
+            | Node::Variant(Variant { ident, .. })
+            | Node::MacroDef(MacroDef { ident, .. })
             | Node::Item(Item { ident, .. }) => Some(*ident),
             _ => None,
         }