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

[Joseph Huber via llvm-commits]

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@@ -213,18 +216,18 @@ Error olShutDown_impl() {
     return Error::success();
 
   llvm::Error Result = Error::success();
+  auto *OldContext = OffloadContextVal.exchange(nullptr);
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jhuber6 wrote:

Yeah, forgot to mention that this needed to be atomic since we're using it outside the mutex as a guard.

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