Decorate some classes and methods with final keyword

This way we silence the clang warnings about calling
virtual methods from c'tor or from d'tor:
[clang-analyzer-optin.cplusplus.VirtualCall]

Change-Id: I8d6318b490152133da4833bda2ba28622bce30dd
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/languageclient/languageclientcompletionassist.cpp

@@ -271,7 +271,7 @@ public:
 };
 
 
-class LanguageClientCompletionAssistProcessor : public IAssistProcessor
+class LanguageClientCompletionAssistProcessor final : public IAssistProcessor
 {
 public:
     LanguageClientCompletionAssistProcessor(Client *client);