[llvm] [Offload] Implement the remaining initial Offload API (PR #122106)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

I think this is fine for now, we can hopefully do some follow-ups in a patch that's less massive. Thanks for sticking with this, at some point I'll need to get some hands-on experience with this so I can make some better judgements. I'm hoping to rewrite the `libc` loader in terms of this API soonish.

https://github.com/llvm/llvm-project/pull/122106