Implement std::experimental::sample.

Following specification in "C++ Extensions for Library Fundamentals":
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4480.html#alg.random.sample



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@237264 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/experimental/algorithms/header.algorithm.synop/includes.pass.cpp

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <experimental/algorithm>
+
+#include <experimental/algorithm>
+
+#ifndef _LIBCPP_ALGORITHM
+#  error "<experimental/algorithm> must include <algorithm>"
+#endif
+
+int main()
+{
+}

test/libcxx/experimental/algorithms/version.pass.cpp

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <experimental/algorithm>
+
+#include <experimental/algorithm>
+
+#ifndef _LIBCPP_VERSION
+#  error _LIBCPP_VERSION not defined
+#endif
+
+int main()
+{
+}