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

[Callum Fare via llvm-commits]

callumfare wrote:

I've added unit tests. The `LibomptUnitTests` target now builds a `offload.unittests` executable. It optionally takes a `--platform` argument to specify the platform/plugin to use, and otherwise picks a sensible default (i.e. one with an available device).

In general it should be possible to fully parameterize the tests for every available platform/device but I've not gone that far at this stage - when the API is a little more fully formed I'm happy to go back and do that.

Incidentally, I disabled the existing `NextgenPluginsTest.cpp` unit test because it didn't even build and seems like it targets an older version of the plugin interface. Probably could just be deleted?

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