C++ front-end: support attributes with tag-less structs

Seen in GCC's STL.

Author: tautschnig

regression/cpp/type_traits_essentials1/main.cpp

@@ -28,6 +28,10 @@ template<typename T> using x=V<T>;
 
 #ifdef __GNUC__
 using type __attribute__((__nodebug__)) = int;
+template <std::size_t _Align = __alignof__(int)>
+struct __attribute__((__aligned__((_Align))))
+{
+} __align;
 #endif
 
 struct alignas(0x1) t