[llvm] Reland of #108413: [Offload] Introduce offload-tblgen and initial new API implementation (PR #118503)

[Callum Fare via llvm-commits]

callumfare wrote:

Hi @jhuber6, sorry for the spam from the various attempts to get this in. I was caught out by the lack of testing and was relying on my single local configuration for testing. I've now tested release and debug builds, the `check-offload` target (including the new offload-tblgen tests), the offload unit tests on all plugins (which aren't run by any check targets right now), header generation, and the install target.

If there's anything else that is likely to go wrong let me know, otherwise I'd appreciate another review and we can hopefully finally land this properly

https://github.com/llvm/llvm-project/pull/118503