Make ExpnData fields krate and orig_id private

These fields are only used by hygiene serialized, and should not be
accessed by anything outside of `rustc_span`.

compiler/rustc_expand/src/expand.rs

@@ -1020,15 +1020,16 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
         // with exception of the derive container case which is not resolved and can get
         // its expansion data immediately.
         let expn_data = match &kind {
-            InvocationKind::DeriveContainer { item, .. } => Some(ExpnData {
-                parent: self.cx.current_expansion.id,
-                ..ExpnData::default(
+            InvocationKind::DeriveContainer { item, .. } => {
+                let mut expn_data = ExpnData::default(
                     ExpnKind::Macro(MacroKind::Attr, sym::derive),
                     item.span(),
                     self.cx.sess.parse_sess.edition,
                     None,
-                )
-            }),
+                );
+                expn_data.parent = self.cx.current_expansion.id;
+                Some(expn_data)
+            }
             _ => None,
         };
         let expn_id = ExpnId::fresh(expn_data);