Enable -Wunreachable-code and fix duplicate warning flags

llvm-svn: 290486
diff --git a/libcxx/test/std/thread/futures/futures.async/async.pass.cpp b/libcxx/test/std/thread/futures/futures.async/async.pass.cpp
index 0f5ee05..9adebb2 100644
--- a/libcxx/test/std/thread/futures/futures.async/async.pass.cpp
+++ b/libcxx/test/std/thread/futures/futures.async/async.pass.cpp
@@ -72,7 +72,8 @@
 void f5(int j)
 {
     std::this_thread::sleep_for(ms(200));
-    TEST_THROW(j); ((void)j);
+    ((void)j);
+    TEST_THROW(j);
 }
 
 template <class Ret, class CheckLamdba, class ...Args>