Avoid unused warning in __throw_future_error w/o exceptions enabled

llvm-svn: 290479
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 82ad195ad559120b5e71c08a89f1b9bb9ef59656
diff --git a/include/future b/include/future
index ce22c1e..d1d42f6 100644
--- a/include/future
+++ b/include/future
@@ -520,6 +520,7 @@
 #ifndef _LIBCPP_NO_EXCEPTIONS
     throw future_error(make_error_code(_Ev));
 #else
+    ((void)_Ev);
     _VSTD::abort();
 #endif
 }