Change QTimeZonePrivate::invalidSeconds() return type from qint64 to int

Inspected all in-tree callers, doesn't change semantics, but fixes a
few -Wshorten-64-to-32 Clang warnings (which we unfortunately didn't
enable globally upon s/int/qsizetype/).

Amends 48e2c3ac33.

Not picking back further than 6.8, because I don't want to review
historic callers.

Task-number: QTBUG-126219
Change-Id: I53c16d630dfffdba3801bf211f1b80d0346dd375
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit 4bf1ed802151f8310b9adb56fac6de93f195cc44)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/time/qtimezoneprivate_p.h

@@ -125,7 +125,7 @@ public:
     { return (std::numeric_limits<qint64>::min)() + 1; }
     [[nodiscard]] static constexpr qint64 invalidMSecs()
     { return (std::numeric_limits<qint64>::min)(); }
-    [[nodiscard]] static constexpr qint64 invalidSeconds()
+    [[nodiscard]] static constexpr int invalidSeconds()
     { return (std::numeric_limits<int>::min)(); }
     static QTimeZone::OffsetData invalidOffsetData();
     static QTimeZone::OffsetData toOffsetData(const Data &data);