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

[Callum Fare via llvm-commits]

callumfare wrote:

Hi @jplehr, I've added a fix for the shared libs build, see the last commit. It's unfortunate that there's a dependency on `FrontendOpenMP`, but I think there would need to be a big refactor to remove all openmp dependencies from offload.

I've also verified a standalone build, including the `check-offload` and `LibomptUnitTests` targets, to make sure this doesn't cause a regression after #118173.

Let me know if there are many issues

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