Allow types to come after consts in AST validation

src/librustc_ast_passes/ast_validation.rs

@@ -735,6 +735,7 @@ fn validate_generic_param_order<'a>(
         }
         let max_param = &mut max_param;
         match max_param {
+            Some(ParamKindOrd::Const) if ParamKindOrd::Type == kind => (),
             Some(max_param) if *max_param > kind => {
                 let entry = out_of_order.entry(kind).or_insert((*max_param, vec![]));
                 entry.1.push(span);