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

[Callum Fare via llvm-commits]

callumfare wrote:

> > @jhuber6 Thanks! I've pushed some small fixes. Would you mind landing this PR when you get a chance? I don't have write access. No new libraries or lit tests this time so hopefully won't be as much trouble as the last one.
> 
> I'll probably want to try testing it locally first. How do you test it?

>From the runtime build dir:
```sh
ninja LibomptUnitTests
./offload/unittests/OffloadAPI/offload.unittests --platform=AMDGPU
```

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