Simplify QDateTimeParser::fromString() to always record the date-time

Previously, *datetime was only written to if the parse was a success.
When parsing a date-time that's invalid by virtue of falling in a
spring-forward gap, the parser returns a date-time that is invalid but
has a toMSecsSinceEpoch() suitable for use in creating a sensible
interpretation of the parsed string (in offset by the width of the gap
from the specified position in the gap). It is more useful to return
this value than a default-created QDateTime.

Change-Id: I89f39e729b1f9fede1532d8b82f6f676477ddadb
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/corelib/time/qdatetimeparser.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2020 The Qt Company Ltd.
+** Copyright (C) 2021 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -2132,15 +2132,9 @@ bool QDateTimeParser::fromString(const QString &t, QDateTime* datetime) const
 {
     QDateTime val(QDate(1900, 1, 1).startOfDay());
     const StateNode tmp = parse(t, -1, val, false);
-    if (tmp.state != Acceptable || tmp.conflicts)
-        return false;
-    if (datetime) {
-        if (!tmp.value.isValid())
-            return false;
+    if (datetime)
         *datetime = tmp.value;
-    }
-
-    return true;
+    return tmp.state == Acceptable && !tmp.conflicts && tmp.value.isValid();
 }
 
 QDateTime QDateTimeParser::getMinimum() const