Clang: Avoid blocking UI if references are requested and clangbackend crashes

The two cases in which the UI might be blocked are:
 * Invocation of the editor context menu
 * Rename Symbol Under Cursor

Cancel the future if the backend is restarted and on the other side,
check whether the future is cancelled.

Change-Id: If2315da1f66f15eab1531fcd8da1dff851a9a4e6
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppuseselectionsupdater.cpp

@@ -98,8 +98,12 @@ void CppUseSelectionsUpdater::update(CallType callType)
 
         // QFuture::waitForFinished seems to block completely, not even
         // allowing to process events from QLocalSocket.
-        while (!future.isFinished())
+        while (!future.isFinished()) {
+            if (future.isCanceled())
+                return;
+
             QCoreApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
+        }
 
         processResults(future.result());
     }