[llvm] Reland #118503: [Offload] Introduce offload-tblgen and initial new API implementation (PR #118614)

[Callum Fare via llvm-commits]

callumfare wrote:

Hi @nikic, for offload-tblgen I mostly copied how mlir-tblgen is set up but I didn't account for the differences in building the project as a runtime. I'll look into this asap but if you any pointers on how to resolve it I'd appreciate the help

https://github.com/llvm/llvm-project/pull/118614