[llvm] [Offload] Improve `olDestroyQueue` logic (PR #153041)

[Ross Brunton via llvm-commits]

================
@@ -47,19 +47,85 @@ struct ol_device_impl_t {
                    ol_platform_handle_t Platform, InfoTreeNode &&DevInfo)
       : DeviceNum(DeviceNum), Device(Device), Platform(Platform),
         Info(std::forward<InfoTreeNode>(DevInfo)) {}
+
+  ~ol_device_impl_t() {
+    assert(!OutstandingQueues.size() &&
+           "Device object dropped with outstanding queues");
+  }
+
   int DeviceNum;
   GenericDeviceTy *Device;
   ol_platform_handle_t Platform;
   InfoTreeNode Info;
+
+  llvm::SmallVector<__tgt_async_info *> OutstandingQueues;
+  std::mutex OutstandingQueuesMutex;
+
+  /// If the device has any outstanding queues that are now complete, remove it
+  /// from the list and return it.
+  ///
+  /// Queues may be added to the outstanding queue list by olDestroyQueue if
+  /// they are destroyed but not completed.
+  std::optional<__tgt_async_info *> GetOutstandingQueue() {
----------------
RossBrunton wrote:

I have my grievances about nullptr being used to represent no value, but elsewhere we seem to use it that way, so I've removed the optional in favour of using nullptr.

https://github.com/llvm/llvm-project/pull/153041