[array.tuple]/1 says that instantiating tuple_element<N, array<T, M>> is ill-formed if N >= M. We didn't do that. Add a static_assert to cause a failure, and a test that checks that we failed

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305191 91177308-0d34-0410-b5e6-96231b3b80d8

include/array

@@ -296,6 +296,7 @@ class _LIBCPP_TEMPLATE_VIS tuple_size<array<_Tp, _Size> >
 template <size_t _Ip, class _Tp, size_t _Size>
 class _LIBCPP_TEMPLATE_VIS tuple_element<_Ip, array<_Tp, _Size> >
 {
+    static_assert(_Ip < _Size, "Index out of bounds in std::tuple_element<> (std::array)");
 public:
     typedef _Tp type;
 };