LanguageClient/ClangCodeModel: Reimplement matchers()

Don't assert when ClientRequestTask::preStartCheck() failed.

Change-Id: I157fe8fc15aa055b70823d6a54454c6eec692f88
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/languageclient/clientrequesttask.h

@@ -14,7 +14,6 @@
 
 namespace LanguageClient {
 
-
 template <typename Request>
 class LANGUAGECLIENT_EXPORT ClientRequestTask
 {
@@ -32,8 +31,10 @@ public:
     void start()
     {
         QTC_ASSERT(!isRunning(), return);
-        QTC_ASSERT(preStartCheck(), m_callback({}); return);
-
+        if (!preStartCheck()) {
+            m_callback({});
+            return;
+        }
         Request request(m_params);
         request.setResponseCallback([this](const typename Request::Response &response) {
             m_response = response;