[libcxx] Add <experimental/ratio>

Summary:
This patch is pretty simple. It just adds the _v traits from <ratio>. 

The draft can be found here.

Reviewers: jroelofs, K-ballo, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7351

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@229509 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/utilities/ratio/header.ratio.synop/includes.pass.cpp

@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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/ratio>
+
+// Test that <ratio> is included.
+
+#include <experimental/ratio>
+
+int main()
+{
+  std::ratio<100> x;
+}