Fix warnings about possible detach of temporary objects

Calling first() or last() on temporary container may
unnecessarily detach the container. Fix it by calling
constFirst() and constLast().

Change-Id: I2460efd5dbee1534eec8a514d9bff2a947bfddf9
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/tools/clangbackend/clangbackendmain.cpp

@@ -53,7 +53,7 @@ QString processArguments(QCoreApplication &application)
     if (parser.positionalArguments().isEmpty())
         parser.showHelp(1);
 
-    return parser.positionalArguments().first();
+    return parser.positionalArguments().constFirst();
 }
 
 #ifdef Q_OS_WIN