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

[Callum Fare via llvm-commits]

callumfare wrote:

 > `libomptarget` is currently written in terms of the `GenericPluginTy` interface, and presumably this API can be as well. The main issue I have with that is there's a lot of OpenMP specific stuff in the plugin that prevents me from adequately fixing / modifying the interface. There's also the fact that modifying stuff upstream tends to conflict with AMD's fork (mostly our fault however).

Ok, I was getting confused between the `GenericPluginTy` interface (which we can presumably change as much as is needed to enable the new API) and the plugin-specific bits of libomptarget (`__tgt_rtl_*`) which I'm assuming we can't change without breaking downstream projects.

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