[orc-rt] Add a make_scope_exit utility.

[lhames]

make_scoped_exit takes a function object and returns an object that will run the given function object at destruction time (unless the scope-exit object's release method is called to disable it).

This can be useful when a function needs to conditionally perform some cleanup, but may exit along multiple pathways. E.g.

  // Allocate resource here.
  auto DoCleanup = make_scope_exit([]() {
    // release resource here.
  });

  // various conditional error return paths.
  if (...)
    return make_error<...>(...);

  // successful return: cancel cleanup.
  DoCleanup.release();
  return Error::success();