[dataflow] Fix complie on gcc7 part 2

Change c2bb680 triggered a bug in gcc7 by disabling a copy
constructor. Commit 2f7d30d fixed one occurrence of this issue, but
after that the compilation still failed with another very similar error.

This commit eliminates this second compilation error, and ensures that
`ninja check-clang-analysis` can be successfully executed with gcc 7.5.

clang/include/clang/Analysis/FlowSensitive/DataflowAnalysis.h

@@ -235,7 +235,7 @@ runDataflowAnalysis(
                   std::move(State.Env)};
             });
       });
-  return BlockStates;
+  return std::move(BlockStates);
 }
 
 /// Abstract base class for dataflow "models": reusable analysis components that