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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> 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.

The `__tgt_rtl_*` functions used to be a pseudo-API because the plugins were all `.so` files loaded via `dlopen`. I reworked that but never go around to modifying the functions themselves.

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