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

[Callum Fare via llvm-commits]

callumfare wrote:

> Generally I'm in favor of `ol_` as a prefix, `snake_case` for the C API, and `get_x` as the verbage.

I'll update the prefix etc. Just wondering though do we want to prefer `olCamelCase` instead to better match the LLVM style rather than just do what omp does? IMO snake_case looks odd unless function parameters also use snake_case, and at that point it kind of pollutes the implementation code with that style

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