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.cpp

@@ -697,6 +697,10 @@ IAssistProposal *ClangCompletionAssistProcessor::createFunctionHintProposal(
     return new FunctionHintProposal(m_positionForProposal, model);
 }
 
+void ClangCompletionAssistProcessor::cancel()
+{
+    m_interface->communicator().cancelCompletions(this);
+}
+
 } // namespace Internal
 } // namespace ClangCodeModel
-