Mojo C++ bindings: extract code shared by different variants.

mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_declaration.tmpl

-{% from "enum_macros.tmpl" import enum_decl -%}
-
 class {{export_attribute}} {{struct.name}} {
  public:
   using DataView = {{struct.name}}DataView;
   using Data_ = internal::{{struct.name}}_Data;
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
-{%-   if enum|is_native_only_kind %}
-  using {{enum.name}} = mojo::NativeEnum;
-{%-   else %}
-  {{enum_decl(enum)|indent(2)}}
-{%-   endif %}
+  using {{enum.name}} = {{enum|get_name_for_kind(flatten_nested_kind=True)}};
 {%- endfor %}
 
 {#--- Constants #}
 {%- for constant in struct.constants %}
 {%-   if constant.kind|is_integral_kind %}
   static const {{constant.kind|cpp_pod_type}} {{constant.name}} = {{constant|constant_value}};
 {%-   else %}
   static const {{constant.kind|cpp_pod_type}} {{constant.name}};
 {%-   endif %}
 {%- endfor %}
@@ skipping 45 matching lines @@
   }
 
 {#--- Struct members #}
 {%  for field in struct.fields %}
 {%-   set type = field.kind|cpp_wrapper_type %}
 {%-   set name = field.name %}
   {{type}} {{name}};
 {%- endfor %}
 };