Add <experimental/coroutine>

This patch adds the library portions of the coroutines PDTS,
which should now be supported by Clang.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@303836 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/double_include.sh.cpp

@@ -137,6 +137,9 @@
 #include <experimental/algorithm>
 #include <experimental/any>
 #include <experimental/chrono>
+#if defined(__cpp_coroutines)
+#include <experimental/coroutine>
+#endif
 #include <experimental/deque>
 #include <experimental/dynarray>
 #include <experimental/filesystem>

test/libcxx/experimental/language.support/support.coroutines/version.sh.cpp

@@ -0,0 +1,25 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     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
+// REQUIRES: fcoroutines-ts
+
+// RUN: %build -fcoroutines-ts
+// RUN: %run
+
+#include <experimental/coroutine>
+
+#ifndef _LIBCPP_VERSION
+#error _LIBCPP_VERSION must be defined
+#endif
+
+int main()
+{
+}