[SYCL] Join thread pool threads on Scheduler destruction

Threads from default queue thread pool can access scheduler after its destruction has begun.
To avoid such illegal access, wait for all threads to join in scheduler destructor.

Author: Alexander Batashev

sycl/source/detail/queue_impl.hpp

@@ -366,6 +366,12 @@ class queue_impl {
     return *MHostTaskThreadPool;
   }
 
+  void stopThreadPool() {
+    if (MHostTaskThreadPool) {
+      MHostTaskThreadPool->finishAndWait();
+    }
+  }
+
   /// Gets the native handle of the SYCL queue.
   ///
   /// \return a native handle.

sycl/source/detail/scheduler/scheduler.cpp

@@ -306,6 +306,7 @@ Scheduler::Scheduler() {
 }
 
 Scheduler::~Scheduler() {
+  DefaultHostQueue->stopThreadPool();
   // By specification there are several possible sync points: buffer
   // destruction, wait() method of a queue or event. Stream doesn't introduce
   // any synchronization point. It is guaranteed that stream is flushed and