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.cpp

@@ -406,6 +406,11 @@ ClangdTextMark::ClangdTextMark(const FilePath &filePath,
         ClangDiagnosticManager::addTask(m_diagnostic);
     }
 
+    m_clientDeleted = QObject::connect(client, &QObject::destroyed, [this] (){
+        QTC_ASSERT_STRING("ClangdClient deleted before TextMark");
+        delete this;
+    });
+
     // Copy to clipboard action
     QVector<QAction *> actions;
     QAction *action = new QAction();
@@ -433,6 +438,11 @@ ClangdTextMark::ClangdTextMark(const FilePath &filePath,
     setActions(actions);
 }
 
+ClangdTextMark::~ClangdTextMark()
+{
+    QObject::disconnect(m_clientDeleted);
+}
+
 bool ClangdTextMark::addToolTipContent(QLayout *target) const
 {
     const auto canApplyFixIt = [c = m_client, diag = m_lspDiagnostic, fp = fileName()] {