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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

We have tblgen tests. Could you do something similar for Offload?
https://github.com/llvm/llvm-project/tree/main/llvm/test/TableGen

Could the tablgen emitter wrap everything into a `namespace offload` to avoid prefixes?

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