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/clangbackendcommunicator.cpp

@@ -513,5 +513,10 @@ void BackendCommunicator::requestCompletions(ClangCompletionAssistProcessor *ass
     m_receiver.addExpectedCompletionsMessage(message.ticketNumber, assistProcessor);
 }
 
+void BackendCommunicator::cancelCompletions(TextEditor::IAssistProcessor *processor)
+{
+    m_receiver.cancelProcessor(processor);
+}
+
 } // namespace Internal
 } // namespace ClangCodeModel