[llvm] [Offload] Implement event sync in amdgpu (PR #149300)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

@jhuber6 I know you said you were going to look into this, but I had a quick look and it seems that all of the machinery was already there, and it just needed plumbing in. Hopefully this is all correct.

https://github.com/llvm/llvm-project/pull/149300