Revert "Remove unused variable."

This reverts commit r194485.

The variable is unused in some macro instantiations, but not others. We should
probably fix clang to not warn on this.

llvm-svn: 194486

llvm/include/llvm/Support/YAMLTraits.h

@@ -1069,6 +1069,7 @@ operator<<(Output &yout, T &seq) {
           seq.resize(index+1);                                              \
         return seq[index];                                                  \
       }                                                                     \
+      static const bool flow = true;                                        \
     };                                                                      \
   }                                                                         \
   }