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.h

@@ -43,6 +43,8 @@ class IEditor;
 class IDocument;
 }
 
+namespace TextEditor { class IAssistProcessor; }
+
 namespace ClangCodeModel {
 namespace Internal {
 
@@ -88,6 +90,7 @@ public:
                             quint32 column,
                             qint32 funcNameStartLine = -1,
                             qint32 funcNameStartColumn = -1);
+    void cancelCompletions(TextEditor::IAssistProcessor *processor);
     void requestAnnotations(const ClangBackEnd::FileContainer &fileContainer);
     QFuture<CppTools::CursorInfo> requestReferences(
             const FileContainer &fileContainer,