Refactor ModuleToObject to offer more flexibility to subclass (NFC)

Some specific implementation of the offload may want more customization, and
even avoid using LLVM in-tree to dispatch the ISA translation to a custom
solution. This refactoring makes it possible for such implementation to work
without even configuring the target backend in LLVM.

Reviewers: fabianmcg

Reviewed By: fabianmcg

Pull Request: https://github.com/llvm/llvm-project/pull/71165