LanguageClient: Track quick fix assist processor

Same as for the completion and function hint processor, these processors
need to be tracked so they get cleaned up on client destruction.

Change-Id: Ib24eb8c652e7a44d8b79e1edddda9ad659d145a3
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/languageclient/languageclientquickfix.cpp

@@ -117,6 +117,7 @@ IAssistProposal *LanguageClientQuickFixAssistProcessor::perform(const AssistInte
         handleCodeActionResponse(response);
     });
 
+    m_client->addAssistProcessor(this);
     m_client->requestCodeActions(request);
     m_currentRequest = request.id();
     return nullptr;