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.cpp

@@ -1310,6 +1310,11 @@ bool CppModelManager::isCppEditor(Core::IEditor *editor)
     return editor->context().contains(ProjectExplorer::Constants::CXX_LANGUAGE_ID);
 }
 
+bool CppModelManager::supportsOutline(const TextEditor::TextDocument *document)
+{
+    return instance()->d->m_activeModelManagerSupport->supportsOutline(document);
+}
+
 bool CppModelManager::isClangCodeModelActive() const
 {
     return d->m_activeModelManagerSupport != d->m_builtinModelManagerSupport;