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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194486 91177308-0d34-0410-b5e6-96231b3b80d8

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;                                        \
     };                                                                      \
   }                                                                         \
   }