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/cppmodelmanagersupport.h

@@ -61,6 +61,7 @@ public:
     virtual FollowSymbolInterface &followSymbolInterface() = 0;
     virtual RefactoringEngineInterface &refactoringEngineInterface() = 0;
     virtual std::unique_ptr<AbstractOverviewModel> createOverviewModel() = 0;
+    virtual bool supportsOutline(const TextEditor::TextDocument *) const { return true; }
 };
 
 class CPPTOOLS_EXPORT ModelManagerSupportProvider