Give LegacyUncomparable::Unordered's value a symbolic name

We'll need it higher up in the file, but we don't want to/can't move
code around, so make -127 a symbolic constant.

Amends 39c7bf4631b401c681c24bb5a732a0359c8a6d7c, which, however,
merely moved the code around.

Pick-to: 6.8
Change-Id: Id6f2d51e5678b3bad7ae658cc3e38fd3909861d7
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 57aabff91eed41fb7ba0cf67df4fe7767c6e0e43)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/qcompare.h

@@ -23,6 +23,7 @@ QT_BEGIN_NAMESPACE
 
 namespace QtPrivate {
 using CompareUnderlyingType = qint8;
+constexpr CompareUnderlyingType LegacyUncomparableValue = -127; // historic Qt value
 
 // [cmp.categories.pre] / 1
 enum class Ordering : CompareUnderlyingType
@@ -675,7 +676,7 @@ auto qCompareThreeWay(const LT &lhs, const RT &rhs)
 namespace QtPrivate {
 enum class LegacyUncomparable : CompareUnderlyingType
 {
-    Unordered = -127
+    Unordered = QtPrivate::LegacyUncomparableValue
 };
 }