[llvm] Reland - [Offload] Introduce offload-tblgen and initial new API implementation (#108413) (PR #117704)

[Callum Fare via llvm-commits]

callumfare wrote:

Hi @jhuber6 - thanks for merging the original PR and apologies for the build problems. My branch was a little behind and I missed some tablegen changes. The last commit in this PR contains the fixes, and things are otherwise identical. I'd appreciate a review on this and getting it relanded, thanks!

https://github.com/llvm/llvm-project/pull/117704