Add <optional> header.

This patch is largely thanks to Casey Carter @ Microsoft. He did the initial
work of porting our experimental implementation and tests over to namespace
std.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@283977 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/optional/optional.relops/not_equal.pass.cpp

@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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
+// <optional>
+
+// template <class T> constexpr bool operator!=(const optional<T>& x, const optional<T>& y);
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+using std::optional;
+
+struct X
+{
+    int i_;
+
+    constexpr X(int i) : i_(i) {}
+};
+
+constexpr bool operator != ( const X &lhs, const X &rhs )
+    { return lhs.i_ != rhs.i_ ; }
+
+int main()
+{
+    {
+    typedef X T;
+    typedef optional<T> O;
+
+    constexpr O o1;     // disengaged
+    constexpr O o2;     // disengaged
+    constexpr O o3{1};  // engaged
+    constexpr O o4{2};  // engaged
+    constexpr O o5{1};  // engaged
+
+    static_assert ( !(o1 != o1), "" );
+    static_assert ( !(o1 != o2), "" );
+    static_assert (  (o1 != o3), "" );
+    static_assert (  (o1 != o4), "" );
+    static_assert (  (o1 != o5), "" );
+
+    static_assert ( !(o2 != o1), "" );
+    static_assert ( !(o2 != o2), "" );
+    static_assert (  (o2 != o3), "" );
+    static_assert (  (o2 != o4), "" );
+    static_assert (  (o2 != o5), "" );
+
+    static_assert (  (o3 != o1), "" );
+    static_assert (  (o3 != o2), "" );
+    static_assert ( !(o3 != o3), "" );
+    static_assert (  (o3 != o4), "" );
+    static_assert ( !(o3 != o5), "" );
+
+    static_assert (  (o4 != o1), "" );
+    static_assert (  (o4 != o2), "" );
+    static_assert (  (o4 != o3), "" );
+    static_assert ( !(o4 != o4), "" );
+    static_assert (  (o4 != o5), "" );
+
+    static_assert (  (o5 != o1), "" );
+    static_assert (  (o5 != o2), "" );
+    static_assert ( !(o5 != o3), "" );
+    static_assert (  (o5 != o4), "" );
+    static_assert ( !(o5 != o5), "" );
+
+    }
+}