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/clangbackendreceiver.h

@@ -35,7 +35,10 @@
 #include <QPointer>
 #include <QTextDocument>
 
-namespace TextEditor { class TextEditorWidget; }
+namespace TextEditor {
+class IAssistProcessor;
+class TextEditorWidget;
+} // namespace TextEditor
 
 namespace ClangCodeModel {
 namespace Internal {
@@ -52,6 +55,7 @@ public:
     void setAliveHandler(const AliveHandler &handler);
 
     void addExpectedCompletionsMessage(quint64 ticket, ClangCompletionAssistProcessor *processor);
+    void cancelProcessor(TextEditor::IAssistProcessor *processor);
     void deleteProcessorsOfEditorWidget(TextEditor::TextEditorWidget *textEditorWidget);
 
     QFuture<CppTools::CursorInfo>