Exploit C++17 init-statements in if to simplify a loop

The serialization converter example's text converter's loadFile() can
be made tidier by making the conditions within its loop into a chain.

Task-number: QTBUG-111228
Change-Id: Ic82355eab7380a0c671b3805ca140958bb1c5af5
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Øystein Heskestad <oystein.heskestad@qt.io>
(cherry picked from commit d631f888042e3f3f5d4d0284c6bad8eb91e2c9a3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/corelib/serialization/convert/textconverter.cpp

@@ -79,21 +79,14 @@ QVariant TextConverter::loadFile(QIODevice *f, Converter *&outputConverter)
     QString line;
     while (!in.atEnd()) {
         in.readLineInto(&line);
-
         bool ok;
-        qint64 v = line.toLongLong(&ok);
-        if (ok) {
+
+        if (qint64 v = line.toLongLong(&ok); ok)
             list.append(v);
-            continue;
-        }
-
-        double d = line.toDouble(&ok);
-        if (ok) {
+        else if (double d = line.toDouble(&ok); ok)
             list.append(d);
-            continue;
-        }
-
-        list.append(line);
+        else
+            list.append(line);
     }
 
     return list;