[llvm] [Offload] Introduce offload-tblgen and initial new API implementation (PR #108413)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Also don't call it new. This would be the... fourth? plugin revision and I think most of them are called some variant on "new".

Honestly my preferred solution would just be to start this as an entirely separate interface so we don't need to deal with breaking OpenMP stuff for existing customers. That's what I wanted from `offload/` in the first place.

https://github.com/llvm/llvm-project/pull/108413