Add [[nodiscard]] to std::async as part of P0600.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318889 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/new b/include/new
index 6541e02..04355e6 100644
--- a/include/new
+++ b/include/new
@@ -271,7 +271,7 @@
 _LIBCPP_NODISCARD_AFTER_CXX17 inline _LIBCPP_INLINE_VISIBILITY
 constexpr _Tp* launder(_Tp* __p) noexcept
 {
-    return __launder(__p);
+    return _VSTD::__launder(__p);
 }
 #endif