Clang: Add job queue for each document

...in preparation for follow-up changes. This will enable e.g. a timer
per document.

This does not change any behavior yet.

Change-Id: Ic1dc06de602373c666d47ce7a95ab99e56d389d5
Reviewed-by: David Schulz <david.schulz@qt.io>

src/tools/clangbackend/ipcsource/clangasyncjob.h

@@ -64,6 +64,11 @@ public:
         return future;
     }
 
+    void preventFinalization() override
+    {
+        m_futureWatcher.disconnect();
+    }
+
 private:
     Runner m_runner;
     QFutureWatcher<Result> m_futureWatcher;