ClangCodeModel: Make use of clangd's "switchSourceHeader" extension

This allows us to switch between headers and sources with different base
names and/or locations, using symbol matching heuristics.

Task-number: QTCREATORBUG-16385
Change-Id: I2d9c07f412d70b75322ed65d491982d78674483d
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangmodelmanagersupport.h

@@ -97,6 +97,7 @@ private:
                       const QString &replacement) override;
     void findUsages(const CppEditor::CursorInEditor &cursor,
                     CppEditor::UsagesCallback &&callback) const override;
+    void switchHeaderSource(const Utils::FilePath &filePath, bool inNextSplit) override;
 
     void onEditorOpened(Core::IEditor *editor);
     void onCurrentEditorChanged(Core::IEditor *newCurrent);