[llvm] [Offload] Implement `olShutDown` and remove global reference counting (PR #144055)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

@jhuber6 Ah, okay I'll add that then. What about calling `olInit` after `olShutDown`? Should that reinitialise everything?

https://github.com/llvm/llvm-project/pull/144055