[dataflow] Fix complie on gcc7 part 2

Change c2bb6807 triggered a bug in gcc7 by disabling a copy
constructor. Commit 2f7d30de 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.