[libc++] Implement LWG 2911 - add an is_aggregate type-trait

Summary:
This patch implements http://cplusplus.github.io/LWG/lwg-defects.html#2911.

I'm putting this up for review until __is_aggregate is added to clang (See D31513)

Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31515

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300126 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/utilities/meta/meta.unary/meta.unary.prop/missing_is_aggregate_trait.fail.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+
+// <type_traits>
+
+// template <class T> struct is_aggregate;
+// template <class T> constexpr bool is_aggregate_v = is_aggregate<T>::value;
+
+#include <type_traits>
+
+int main ()
+{
+#ifdef _LIBCPP_HAS_NO_IS_AGGREGATE
+  // This should not compile when _LIBCPP_HAS_NO_IS_AGGREGATE is defined.
+  bool b = __is_aggregate(void);
+  ((void)b);
+#else
+#error Forcing failure...
+#endif
+}