[llvm] [offload] add nodiscard support to offload-tblgen and mark olInit (PR #209727)

[Łukasz Plewa via llvm-commits]

lplewa wrote:

I have mix feeling about this change. I'm not sure if we should introduce things from newer standards conditionally. But I'm also not against this patch. But this is something I would like the maintainers to review.

@jdoerfert @jhuber6 

https://github.com/llvm/llvm-project/pull/209727