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/clangdclient.h

@@ -75,6 +75,7 @@ public:
             const QTextCursor &cursor,
             CppEditor::CppEditorWidget *editorWidget,
             Utils::ProcessLinkCallback &&callback);
+    void switchHeaderSource(const Utils::FilePath &filePath, bool inNextSplit);
 
     void findLocalUsages(TextEditor::TextDocument *document, const QTextCursor &cursor,
                          CppEditor::RenameCallback &&callback);