fix two warnings from clang

[svigerske]

1. The clone() method of a CoinMessageHandler is const. To overwrite it properly, make also SHOT's variant const.
2. There is a std::move which apparently is useless, but generates a warning:

   Problem.cpp:2605:23: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
               ->add(std::move(copyNonlinearExpression(
                     ^
   Problem.cpp:2605:23: note: remove std::move call here
               ->add(std::move(copyNonlinearExpression(
                     ^~~~~~~~~~

   (https://stackoverflow.com/questions/70752203/stdmove-versus-copy-elision)