QPartialOrdering: mark isOrdered() as const

Because it is.

Amends 405244fe301ac18d20aae245ba2faafaec74e453.

Pick-to: 6.5
Change-Id: Icc7bb1a000e89b63254ca5c306325d01603eb1d4
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit ccc46d8f89226b6a89f7c1a63e44d4605483b1cb)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qcompare.h

@@ -107,7 +107,7 @@ private:
 
     // instead of the exposition only is_ordered member in [cmp.partialord],
     // use a private function
-    constexpr bool isOrdered() noexcept
+    constexpr bool isOrdered() const noexcept
     { return m_order != static_cast<QtPrivate::CompareUnderlyingType>(QtPrivate::Uncomparable::Unordered); }
 
     QtPrivate::CompareUnderlyingType m_order;