[llvm] [OFFLOAD][OMP] Move OpenMP kernel argument processing from plugins (PR #213867)

[Łukasz Plewa via llvm-commits]

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lplewa wrote:

NIT: we should resize(0) those vectors anyway if NumArgs == 0. 
As user could assume that Args.len() == NumArgs. 

(i know that those vectors are empty - but this whould be more "defensive" approach if the calling code changed)

https://github.com/llvm/llvm-project/pull/213867