__threading_support: fix windows build

The build was broken as there was no overload for long and
std::chrono::nanoseconds.  Add an explicit conversion to use the
operator+.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294698 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/__threading_support b/include/__threading_support
index b938d9c..1e8be3f 100644
--- a/include/__threading_support
+++ b/include/__threading_support
@@ -591,7 +591,8 @@
 {
   using namespace chrono;
   // round-up to the nearest milisecond
-  milliseconds __ms = duration_cast<milliseconds>(__ns + 999999);
+  milliseconds __ms =
+      duration_cast<milliseconds>(__ns + chrono::nanoseconds(999999));
   Sleep(__ms.count());
 }