ClangCodeModel: Provide outline via clangd

Note that we used to encode the information about symbol visibility and
static-ness in the icons, which we can't do anymore, because clangd does
not provide this information.
On the upside, this change likely fixes a ton of bugs, as our own outline
was rather "quirky".

Change-Id: I099f11ec4e3c6f52cd461fb43080bbdde3bed5e5
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cppmodelmanager.h

@@ -142,6 +142,7 @@ public:
     void emitAbstractEditorSupportRemoved(const QString &filePath);
 
     static bool isCppEditor(Core::IEditor *editor);
+    static bool supportsOutline(const TextEditor::TextDocument *document);
     bool isClangCodeModelActive() const;
 
     QSet<AbstractEditorSupport*> abstractEditorSupports() const;