unique_ptrify Driver Action handling

It hits a limit when we reach ActionList, which is used for dynamic
conditional ownership, so we lose type safety there.

This did expose at least one caller "lying" about ownership (passing
ownership to an Action, then updating the Action to specify that it
doesn't actually own the thing that was passed) - changing this to
unique_ptr just makes that oddity more obvious.

llvm-svn: 216713