[llvm] [Offload] Implement olMemFill (PR #154102)

[Callum Fare via llvm-commits]

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callumfare wrote:

Yes, they'll fail as-is. @RossBrunton is going to finish implementing AMDGPU support in a follow up PR (I don't have the ability to test on AMD hardware right now). I can wait to merge this until that's ready so there's only a small window where the tests are failing.

https://github.com/llvm/llvm-project/pull/154102