Merge pull request swiftlang#2153 from ahoppen/check-shutdown

Do not schedule any new tasks in `TaskScheduler` if it has been shut down

Author: ahoppen

Sources/SemanticIndex/TaskScheduler.swift

@@ -521,6 +521,9 @@ package actor TaskScheduler<TaskDescription: TaskDescriptionProtocol> {
   ///
   /// This will continue calling itself until the queue is empty.
   private func poke() {
+    if isShutDown {
+      return
+    }
     pendingTasks.sort(by: { $0.priority > $1.priority })
     for task in pendingTasks {
       guard