[ExecutionEngine] Fix warnings

This patch fixes:

  llvm/lib/ExecutionEngine/Orc/ExecutionUtils.cpp:512:12: error:
  moving a temporary object prevents copy elision
  [-Werror,-Wpessimizing-move]

and:

  llvm/lib/ExecutionEngine/Orc/ExecutionUtils.cpp:515:12: error:
  moving a temporary object prevents copy elision
  [-Werror,-Wpessimizing-move]