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

[Callum Fare via llvm-commits]

callumfare wrote:

One other reason to check in the headers is so that we can rely on them being clang-formatted without having a hard dependency on clang-format. I assume this is better for standalone builds and other unusual configurations.

Perhaps as a compromise the user facing headers (`offload_api.h`, and probably `offload_print.hpp`) could be checked in but the other files (which are purely implementation details) left to be auto-generated normally, since we don't particularly care about them being formatted correctly.

I think the reviewability will be less of an issue after this initial PR since future changes will be smaller and reviewers will be more familiar with what gets generated. I still have a preference for checking in the files but I understand it's not how tablegen has been used so far so it's controversial.

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