[coroutines] Allow compilation under c++03

llvm-svn: 329239
diff --git a/libcxx/include/experimental/coroutine b/libcxx/include/experimental/coroutine
index 862d545..fef5e08 100644
--- a/libcxx/include/experimental/coroutine
+++ b/libcxx/include/experimental/coroutine
@@ -294,7 +294,7 @@
 
 inline _LIBCPP_INLINE_VISIBILITY
 noop_coroutine_handle noop_coroutine() _NOEXCEPT {
-  return {};
+  return noop_coroutine_handle();
 }
 #endif // __has_builtin(__builtin_coro_noop)