Clangd: clean up text marks after deleting client

This is a blindshot and more a workaround than a fix.
Assert and try to cleanup dangling text marks after a client got
deleted. In theory those marks should get deleted by the
DiagnosticManager but somehow, they are still alive and happy after the
client was deleted.

Task-number: QTCREATORBUG-26585
Change-Id: I9d5d708db3fbbe30a09d322400d97184fe40a518
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clangcodemodel/clangtextmark.h

@@ -73,6 +73,7 @@ public:
     ClangdTextMark(const ::Utils::FilePath &filePath,
                    const LanguageServerProtocol::Diagnostic &diagnostic,
                    const LanguageClient::Client *client);
+    ~ClangdTextMark();
 
 private:
     bool addToolTipContent(QLayout *target) const override;
@@ -80,6 +81,8 @@ private:
     const LanguageServerProtocol::Diagnostic m_lspDiagnostic;
     const ClangBackEnd::DiagnosticContainer m_diagnostic;
     const QPointer<const LanguageClient::Client> m_client;
+
+    QMetaObject::Connection m_clientDeleted;
 };
 
 } // namespace Internal