mbe: expand_invoc: Add comment about not needing to check safety of LegacyAttr here

`LegacyAttr` is only used for builtin attributes, and builtin attributes
have their safety checked by `check_attribute_safety`, so we don't need
to check `unsafety` here.

Author: joshtriplett

compiler/rustc_expand/src/expand.rs

@@ -842,6 +842,9 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                         Err(guar) => return ExpandResult::Ready(fragment_kind.dummy(span, guar)),
                     }
                 } else if let SyntaxExtensionKind::LegacyAttr(expander) = ext {
+                    // `LegacyAttr` is only used for builtin attribute macros, which have their
+                    // safety checked by `check_builtin_meta_item`, so we don't need to check
+                    // `unsafety` here.
                     match validate_attr::parse_meta(&self.cx.sess.psess, &attr) {
                         Ok(meta) => {
                             let item_clone = macro_stats.then(|| item.clone());