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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> I'm not against having that discussion again, before we start actually committing new code, if it makes thing easier for us. If we keep the current approach it seems important to understand how we actually transfer the existing code (both plugins and libomptarget runtime) to the new API without breaking OpenMP things.

`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).

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