Fix signature of QDebug::toString() (again)

There are zero QDebug operators that take an rvalue RHS, so there's no
need to make toString() perfectly forwarding. It just causes four¹
different versions of the identical function to be instantiated. Not
all compilers can merge the binary representations and those that try,
violate the standard (same address for different functions).

¹ {rvalue, lvalue} × {const, non-const}

Fix by taking by cref.

Amends 40976532158fc49be45bb976455f48e98f9690cf.

[ChangeLog][Potentially Source-Incompatible Changes][QtCore][QDebug]
The toString() static function no longer allows the called
operator<<() to modify the argument. No Qt-provided operator<<() does
that, and operators that do are probably buggy.

Change-Id: Iac1ee9d29f93fd4840b75ffe363d354a2d19a96e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 13244eef6c913bb7f27cd8fbf459f5dfe9a0ed21)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qdebug.cpp

@@ -996,7 +996,7 @@ QDebug &QDebug::resetFormat()
 */
 
 /*!
-    \fn template <class T> QString QDebug::toString(T &&object)
+    \fn template <class T> QString QDebug::toString(const T &object)
     \since 6.0
 
     \include qdebug-toString.qdocinc

src/corelib/io/qdebug.h

@@ -224,11 +224,11 @@ public:
 #endif // QT_SUPPORTS_INT128
 
     template <typename T>
-    static QString toString(T &&object)
+    static QString toString(const T &object)
     {
         QString buffer;
         QDebug stream(&buffer);
-        stream.nospace() << std::forward<T>(object);
+        stream.nospace() << object;
         return buffer;
     }
 };