Fix hard-to-read code in dof_renumbering.cc.

[bangerth]

In looking at dof_renumbering.cc as part of #15966, I found this gem (in two places in the file):

    IndexSet        locally_relevant_dofs;
    const IndexSet &locally_owned_dofs = [&]() -> const IndexSet & {
      if (reorder_level_dofs == false)
        {
          DoFTools::extract_locally_relevant_dofs(dof_handler,
                                                  locally_relevant_dofs);
          return dof_handler.locally_owned_dofs();
        }
      else
        {
          Assert(dof_handler.n_dofs(level) != numbers::invalid_dof_index,
                 ExcDoFHandlerNotInitialized());
          DoFTools::extract_locally_relevant_level_dofs(dof_handler,
                                                        level,
                                                        locally_relevant_dofs);
          return dof_handler.locally_owned_mg_dofs(level);
        }
    }();

This is hard to read because the lambda function seems to compute a value to initialize locally_owned_dofs, but really it also initializes another variable. Surely this can be done in a way that is easier to read :-)

(In fact, after #15966, we can directly initialize both variables because we use versions of the extract_* functions that return the index set, rather than set an argument passed by reference.)