Explicitly convert int to future_errc.  Fixes http://llvm.org/bugs/show_bug.cgi?id=11428

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@149630 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/future.cpp b/src/future.cpp
index d5c55b6..2935711 100644
--- a/src/future.cpp
+++ b/src/future.cpp
@@ -29,7 +29,7 @@
 string
 __future_error_category::message(int ev) const
 {
-    switch (ev)
+    switch (static_cast<future_errc>(ev))
     {
     case future_errc::broken_promise:
         return string("The associated promise has been destructed prior "