ClangCodeModel: Fix issue pane management for clangd diagnostics

We basically forgot to implement this. A task in the issues pane was
created for every text mark, but there was no code to deal with handling
editor switches, resulting in seemingly random behavior.

Fixes: QTCREATORBUG-27260
Change-Id: Ifcc1e04db4c37dde7e80f0e1646dc7c557cd7481
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangtextmark.h

@@ -47,7 +47,7 @@ public:
     ClangdTextMark(const ::Utils::FilePath &filePath,
                    const LanguageServerProtocol::Diagnostic &diagnostic,
                    bool isProjectFile,
-                   const LanguageClient::Client *client);
+                   LanguageClient::Client *client);
 
 private:
     bool addToolTipContent(QLayout *target) const override;