CppEditor: Acknowledge the existence of clangd

Stop pretending to be super generic. Instead, let interested code know
directly whether a document is under clangd's control.
This saves code and makes the logic easier to understand.

Change-Id: Ia19d0ec6c4e83926379a7d17ca53896bee3a50e1
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppeditorwidget.cpp

@@ -404,7 +404,7 @@ class CppEditorWidgetPrivate
 public:
     CppEditorWidgetPrivate(CppEditorWidget *q);
 
-    bool shouldOfferOutline() const { return CppModelManager::supportsOutline(m_cppEditorDocument); }
+    bool shouldOfferOutline() const { return !CppModelManager::usesClangd(m_cppEditorDocument); }
 
 public:
     QPointer<CppModelManager> m_modelManager;