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/clangcodemodel/clangbackendipcintegration.h

@@ -73,14 +73,14 @@ public:
     void setAliveHandler(const AliveHandler &handler);
 
     void addExpectedCodeCompletedMessage(quint64 ticket, ClangCompletionAssistProcessor *processor);
-    void deleteAndClearWaitingAssistProcessors();
     void deleteProcessorsOfEditorWidget(TextEditor::TextEditorWidget *textEditorWidget);
 
     QFuture<CppTools::CursorInfo> addExpectedReferencesMessage(quint64 ticket,
                                                                QTextDocument *textDocument);
-
     bool isExpectingCodeCompletedMessage() const;
 
+    void reset();
+
 private:
     void alive() override;
     void echo(const ClangBackEnd::EchoMessage &message) override;