[clang] [clang][Driver] Honor `--offload-jobs` in device-only compilation (PR #233184)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Honestly, I can't decide if we should bother here. The whole reason we have `--offload-jobs` is because it's difficult to parallelize offloading jobs because they combine one host job with many device jobs. Parallelizing this manually would result in many conflicting host objects. But for `--offload-device-only` it's closer to a straightforward compilation and I'd expect the user to just create multiple jobs for it themselves. Though, I guess it's still annoying to need to invoke the offload bundler at the end or something?

https://github.com/llvm/llvm-project/pull/233184