CPlusPlus: Avoid a few deprecation warnings

The alternatives have been around since 2012.

Change-Id: I0aa15d59efe8a547e2ad622ffda689746960d48c
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/libs/cplusplus/CppDocument.cpp

@@ -228,8 +228,7 @@ public:
         if (fileName != doc->fileName())
             return;
 
-        QString message;
-        message.vsprintf(format, ap);
+        const QString message = QString::vasprintf(format, ap);
 
 #ifndef DO_NOT_DUMP_ALL_PARSER_ERRORS
         {