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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

We have `ninja install-llvm-headers` for a reason. External applications should not build on a mono repo checkout. You have to install llvm before using it.

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