LanguageClient: Do not untrack documents before restarting a client

shutdownClient removed the association between the document and the
client, but this is one of the information we would like to keep when
restarting a client. Also we never want to delete the client when
calling restart.

Change-Id: I0558c27ba4b9171c7933edaf2e17847d4e6f62b6
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/languageclientmanager.cpp

@@ -109,8 +109,11 @@ void LanguageClient::LanguageClientManager::addClient(Client *client)
 void LanguageClientManager::restartClient(Client *client)
 {
     QTC_ASSERT(managerInstance, return);
+    if (!client)
+        return;
     managerInstance->m_restartingClients.insert(client);
-    shutdownClient(client);
+    if (client->reachable())
+        client->shutdown();
 }
 
 void LanguageClientManager::clientStarted(Client *client)