Implement #[define_opaque] attribute for functions.

compiler/rustc_builtin_macros/src/alloc_error_handler.rs

@@ -88,6 +88,7 @@ fn generate_handler(cx: &ExtCtxt<'_>, handler: Ident, span: Span, sig_span: Span
         generics: Generics::default(),
         contract: None,
         body,
+        define_opaque: None,
     }));
 
     let attrs = thin_vec![cx.attr_word(sym::rustc_std_internal_symbol, span)];