Retain struct doc comments from C++ headers.

We should also retain doc comments for functions and fields. That will come in
follow-up CLs.

PiperOrigin-RevId: 400961334
diff --git a/rs_bindings_from_cc/ir.h b/rs_bindings_from_cc/ir.h
index 848bc3c..59ea7f4 100644
--- a/rs_bindings_from_cc/ir.h
+++ b/rs_bindings_from_cc/ir.h
@@ -234,6 +234,7 @@
   nlohmann::json ToJson() const;
 
   Identifier identifier;
+  std::optional<std::string> doc_comment;
   std::vector<Field> fields;
   // Size and alignment in bytes.
   int64_t size;