Support overaligned types in `aligned_storage`.

[EricWF]

Summary

The current implementation of aligned storage was written before we had alignas, so it used a list of builtin types to force the alignment. But this doesn't work overaligned requests.

This patch adds a fallback case supporting over-alignment. It only affects case that were previously ill-formed.

[mclow.lists]

LGTM.

libcxx/trunk/include/type_traits

typedef
     __type_list<__align_type<double>,
     __type_list<__align_type<long double>,
     __type_list<__align_type<__struct_double>,
     __type_list<__align_type<__struct_double4>,
     __type_list<__align_type<int*>,
     __nat
     > > > > > > > > > > __all_types;
 
+template <size_t _Align>
+struct _ALIGNAS(_Align) __fallback_overaligned {};
+
 template <class _TL, size_t _Align> struct __find_pod;
 
 template <class _Hp, size_t _Align>
 struct __find_pod<__type_list<_Hp, __nat>, _Align>
 {
     typedef typename conditional<
                              _Align == _Hp::value,
                              typename _Hp::type,
-                             void
+                             __fallback_overaligned<_Align>
                          >::type type;
 };
 
 template <class _Hp, class _Tp, size_t _Align>
 struct __find_pod<__type_list<_Hp, _Tp>, _Align>
 {
     typedef typename conditional<
                              _Align == _Hp::value,

 template <class _Hp, class _Tp, size_t _Len>
 struct __find_max_align<__type_list<_Hp, _Tp>, _Len>
     : public integral_constant<size_t, __select_align<_Len, _Hp::value, __find_max_align<_Tp, _Len>::value>::value> {};
 
 template <size_t _Len, size_t _Align = __find_max_align<__all_types, _Len>::value>
 struct _LIBCPP_TEMPLATE_VIS aligned_storage
 {
     typedef typename __find_pod<__all_types, _Align>::type _Aligner;
-    static_assert(!is_void<_Aligner>::value, "");
     union type
     {
         _Aligner __align;
         unsigned char __data[(_Len + _Align - 1)/_Align * _Align];
     };
 };
 
 #if _LIBCPP_STD_VER > 11

libcxx/trunk/test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp

 #if TEST_STD_VER > 11
     ASSERT_SAME_TYPE(T1, std::aligned_storage_t<10>);
 #endif
     static_assert(std::is_trivial<T1>::value, "");
     static_assert(std::is_standard_layout<T1>::value, "");
     static_assert(std::alignment_of<T1>::value == 8, "");
     static_assert(sizeof(T1) == 16, "");
     }
+  {
+    const int Align = 65536;
+    typedef typename std::aligned_storage<1, Align>::type T1;
+    static_assert(std::is_trivial<T1>::value, "");
+    static_assert(std::is_standard_layout<T1>::value, "");
+    static_assert(std::alignment_of<T1>::value == Align, "");
+    static_assert(sizeof(T1) == Align, "");
+  }
 
   return 0;
 }