[libcxx] Add <experimental/any> v2.

Summary:
This patch adds the second revision of <experimental/any>. 
I've been working from the LFTS draft found at this link. https://rawgit.com/cplusplus/fundamentals-ts/v1/fundamentals-ts.html#any



Reviewers: danalbert, jroelofs, K-ballo, mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6762

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@243728 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/any/any.class/any.modifiers/clear.pass.cpp

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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
+
+// <experimental/any>
+
+// any::clear() noexcept
+
+#include <experimental/any>
+#include <cassert>
+
+#include "any_helpers.h"
+
+int main()
+{
+    using std::experimental::any;
+    using std::experimental::any_cast;
+    // empty
+    {
+        any a;
+
+        // noexcept check
+        static_assert(
+            noexcept(a.clear())
+          , "any.clear() must be noexcept"
+          );
+
+        assertEmpty(a);
+
+        a.clear();
+
+        assertEmpty(a);
+    }
+    // small object
+    {
+        any a((small(1)));
+        assert(small::count == 1);
+        assertContains<small>(a, 1);
+
+        a.clear();
+
+        assertEmpty<small>(a);
+        assert(small::count == 0);
+    }
+    // large object
+    {
+        any a(large(1));
+        assert(large::count == 1);
+        assertContains<large>(a);
+
+        a.clear();
+
+        assertEmpty<large>(a);
+        assert(large::count == 0);
+    }
+}