[llvm] [Offload] Implement `olShutDown` (PR #144055)

[Ross Brunton via llvm-commits]

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@@ -169,13 +173,16 @@ void initPlugins() {
 }
 
 Error olInit_impl() {
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RossBrunton wrote:

I thought that the reference counting would be sufficient, but thinking about it more, yes a mutex is required. Thanks, it's been added.

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