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

[Johannes Doerfert via llvm-commits]

https://github.com/jdoerfert commented:

I left a bunch of comments in addition to what we discussed earlier.
There were also some copy-n-paste errors in the td files.

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