using directives: allow attributes to appear at the beginning

Author: jdrouhard

grammar.js

@@ -65,6 +65,8 @@ module.exports = grammar(C, {
     [$.sized_type_specifier],
     [$.attributed_statement],
     [$._declaration_modifiers, $.attributed_statement],
+    [$._declaration_modifiers, $.using_declaration],
+    [$._declaration_modifiers, $.attributed_statement, $.using_declaration],
     [$._top_level_item, $._top_level_statement],
     [$._block_item, $.statement],
     [$.type_qualifier, $.extension_expression],
@@ -804,6 +806,7 @@ module.exports = grammar(C, {
     )),
 
     using_declaration: $ => seq(
+      repeat($.attribute_declaration),
       'using',
       optional(choice('namespace', 'enum')),
       choice(

test/corpus/declarations.txt

@@ -98,6 +98,7 @@ using ::e::f::g;
 using h = i::j;
 using namespace std;
 using enum Foo;
+[[deprecated]] using namespace std;
 
 template <typename T>
 using a = typename b<T>::c;
@@ -134,6 +135,11 @@ using foobar [[deprecated]] [[maybe_unused]] = int;
     (identifier))
   (using_declaration
     (identifier))
+  (using_declaration
+    (attribute_declaration
+      (attribute
+        (identifier)))
+    (identifier))
   (template_declaration
     (template_parameter_list
       (type_parameter_declaration