Clang: Fix crash after cancelled completion request

Amends

  commit 755de9f86a
  Editor: delete processor after canceling

Ensure to update the bookkeeping in BackendReceiver on

  CodeAssistantPrivate::cancelCurrentRequest()

as otherwise we run into a use-after-free.

Change-Id: Ic5abdc9d743dc0b76eb050a9e3e00f85d2c5bc96
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangcompletionassistprocessor.h

@@ -56,6 +56,7 @@ public:
     const TextEditor::TextEditorWidget *textEditorWidget() const;
 
 private:
+    void cancel() override;
     TextEditor::IAssistProposal *startCompletionHelper();
     int startOfOperator(int pos, unsigned *kind, bool wantFunctionCall) const;
     int findStartOfName(int pos = -1) const;