[v3] implement LWG 2056

[Jonathan Wakely]

        PR libstdc++/49204
        * include/std/future (future_errc): Implement LWG 2056.

tested x86_84-linux, committed to trunk
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Index: include/std/future
===================================================================
--- include/std/future	(revision 182657)
+++ include/std/future	(revision 182658)
@@ -60,10 +60,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// Error code for futures
   enum class future_errc
   {
-    broken_promise,
-    future_already_retrieved,
+    future_already_retrieved = 1,
     promise_already_satisfied,
-    no_state
+    no_state,
+    broken_promise
   };
 
   /// Specialization.