Fix [-Wpessimizing-move] warning on clang 3.9.0.
Fix [-Wpessimizing-move] warning on clang 3.9.0.

Not sure why this doesn't show up as an error on our -Werror builds,
perhaps it is just a warning on older clangs.

Refs #12424.

The actual text of the warning is below:

TheWarehouse.h:197:67: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
    std::vector<std::unique_ptr<Attribute>> attributes() { return std::move(clone()._attribs); }
                                                                  ^
TheWarehouse.h:197:67: note: remove std::move call here
    std::vector<std::unique_ptr<Attribute>> attributes() { return std::move(clone()._attribs); }
                                                                  ^~~~~~~~~~                ~
1 warning generated.

andrsd/moose,sapitts/moose,permcody/moose,YaqiWang/moose,SudiptaBiswas/moose,milljm/moose,lindsayad/moose,nuclear-wizard/moose,dschwen/moose,nuclear-wizard/moose,bwspenc/moose,milljm/moose,andrsd/moose,YaqiWang/moose,permcody/moose,jessecarterMOOSE/moose,laagesen/moose,sapitts/moose,harterj/moose,laagesen/moose,idaholab/moose,idaholab/moose,milljm/moose,laagesen/moose,bwspenc/moose,jessecarterMOOSE/moose,lindsayad/moose,idaholab/moose,lindsayad/moose,jessecarterMOOSE/moose,harterj/moose,sapitts/moose,andrsd/moose,lindsayad/moose,sapitts/moose,laagesen/moose,SudiptaBiswas/moose,milljm/moose,bwspenc/moose,jessecarterMOOSE/moose,milljm/moose,idaholab/moose,dschwen/moose,jessecarterMOOSE/moose,SudiptaBiswas/moose,idaholab/moose,nuclear-wizard/moose,dschwen/moose,bwspenc/moose,sapitts/moose,permcody/moose,SudiptaBiswas/moose,lindsayad/moose,bwspenc/moose,harterj/moose,harterj/moose,nuclear-wizard/moose,YaqiWang/moose,SudiptaBiswas/moose,andrsd/moose,dschwen/moose,andrsd/moose,permcody/moose,dschwen/moose,YaqiWang/moose,harterj/moose,laagesen/moose