Get <experimental/coroutine> working in C++03.

Clang supports coroutines in all dialects; Therefore libc++ should too,
otherwise the Clang extension is unusable.

I'm not convinced extending support to C++03 is a feasible long term
plan, since as the library grows to offer things like generators it
will be come increasingly difficult to limit the implementation to C++03.

However for the time being supporting C++03 isn't a big deal.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@303963 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -0,0 +1,57 @@
+// -*- 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.
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: fcoroutines-ts
+
+// RUN: %build -fcoroutines-ts
+// RUN: %run
+
+// A simple "breathing" test that checks that <experimental/coroutine>
+// can be parsed and used in all dialects, including C++03 in order to match
+// Clang's behavior.
+
+#include <experimental/coroutine>
+
+namespace coro = std::experimental::coroutines_v1;
+
+coro::suspend_always sa;
+coro::suspend_never sn;
+
+struct MyFuture {
+  struct promise_type {
+    typedef coro::coroutine_handle<promise_type> HandleT;
+    coro::suspend_always initial_suspend() { return sa; }
+    coro::suspend_never final_suspend() { return sn; }
+    coro::suspend_never yield_value(int) { return sn; }
+    MyFuture get_return_object() {
+      MyFuture f(HandleT::from_address(this));
+      return f;
+    }
+    void return_void() {}
+    void unhandled_exception() {}
+  };
+  typedef promise_type::HandleT HandleT;
+  MyFuture() : p() {}
+  MyFuture(HandleT h) : p(h) {}
+
+private:
+  coro::coroutine_handle<promise_type> p;
+};
+
+MyFuture test_coro() {
+  co_await sn;
+  co_yield 42;
+  co_return;
+}
+
+int main()
+{
+  MyFuture f = test_coro();
+}