Clangd: Fix autocompletion for old style SIGNAL and SLOT

Fixes: QTCREATORBUG-20737
Change-Id: If6d3c6ea5924537386eca81d90d4bb1e8f1a1466
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cppeditor/cpptoolsreuse.cpp

@@ -28,6 +28,7 @@
 #include "clangdiagnosticconfigsmodel.h"
 #include "cppautocompleter.h"
 #include "cppcodemodelsettings.h"
+#include "cppcompletionassist.h"
 #include "cppeditorconstants.h"
 #include "cppeditorplugin.h"
 #include "cpphighlighter.h"
@@ -336,6 +337,11 @@ TextEditor::QuickFixOperations quickFixOperations(const TextEditor::AssistInterf
     return Internal::quickFixOperations(interface);
 }
 
+CppCompletionAssistProcessor *getCppCompletionAssistProcessor()
+{
+    return new Internal::InternalCppCompletionAssistProcessor();
+}
+
 CppCodeModelSettings *codeModelSettings()
 {
     return Internal::CppEditorPlugin::instance()->codeModelSettings();