GCC 5.3 build fix

It was failing with

  llvm/lib/ExecutionEngine/Orc/DebugUtils.cpp:56:10:
  error: could not convert ‘Obj’ from ‘std::unique_ptr<llvm::MemoryBuffer>’
  to ‘llvm::Expected<std::unique_ptr<llvm::MemoryBuffer> >’
     return Obj;
            ^