ClangCodeModel: Make clangd refactoring actions available

Introduce an assist processor that merges our built-in quickfixes with
refactoring actions from clangd ("tweaks").
For now, we make it clear which ones are coming from clangd, and we do
not filter duplicate functionality. In the future, we might want to
disable redundant built-in actions if clangd is enabled for the
respective file.

Change-Id: I04842132798c8635dfddf8cfc98cc7a6313fac09
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppeditordocument.cpp

@@ -162,6 +162,8 @@ CompletionAssistProvider *CppEditorDocument::functionHintAssistProvider() const
 
 TextEditor::IAssistProvider *CppEditorDocument::quickFixAssistProvider() const
 {
+    if (const auto baseProvider = TextDocument::quickFixAssistProvider())
+        return baseProvider;
     return CppEditorPlugin::instance()->quickFixProvider();
 }