Property binding system: Fix developer build with MSVC 2019 (16.4.1)

- Fix wrong forward declaration
- De-inline constructor of QUntypedPropertyBinding(), fixing:

qtbase\include/src/corelib/tools/qshareddata.h(184): error C2027: use of undefined type 'QPropertyBindingPrivate'
qtbase\include/src/corelib/kernel/qpropertyprivate.h(60): note: see declaration of 'QPropertyBindingPrivate'
qtbase\include/src/corelib/tools/qshareddata.h(184): note: while compiling class template member function 'QExplicitlySharedDataPointer<QPropertyBindingPrivate>::~QExplicitlySharedDataPointer(void)'
qtbase\src\corelib\kernel\qproperty.h(143): note: see reference to function template instantiation 'QExplicitlySharedDataPointer<QPropertyBindingPrivate>::~QExplicitlySharedDataPointer(void)' being compiled
qtbase\src\corelib\kernel\qproperty.h(142): note: see reference to class template instantiation 'QExplicitlySharedDataPointer<QPropertyBindingPrivate>' being compiled
qtbase\include\/src/corelib/tools/qshareddata.h(184): warning C4150: deletion of pointer to incomplete type 'QPropertyBindingPrivate'; no destructor called
qtbase\include\/src/corelib/kernel/qpropertyprivate.h(60): note: see declaration of 'QPropertyBindingPrivate'

Amends 9f9049b486.

Change-Id: Idd613e2487d5ab7f8ead74747acd976d5d210c28
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/corelib/kernel/qproperty.h

@@ -122,7 +122,7 @@ public:
     // already has.
     using BindingEvaluationFunction = std::function<BindingEvaluationResult(int version, void *propertyStoragePtr)>;
 
-    QUntypedPropertyBinding() = default;
+    QUntypedPropertyBinding();
     QUntypedPropertyBinding(BindingEvaluationFunction function, const QPropertyBindingSourceLocation &location);
     QUntypedPropertyBinding(QUntypedPropertyBinding &&other);
     QUntypedPropertyBinding(const QUntypedPropertyBinding &other);
@@ -337,7 +337,7 @@ private:
     Q_DISABLE_COPY(QProperty)
 
     friend struct QPropertyBasePointer;
-    friend struct QPropertyBinding<T>;
+    friend class QPropertyBinding<T>;
     friend struct QPropertyObserver;
     // Mutable because querying for the value may require evalating the binding expression, calling
     // non-const functions on QPropertyBase.

src/corelib/kernel/qpropertybinding.cpp

@@ -89,6 +89,8 @@ bool QPropertyBindingPrivate::evaluateIfDirtyAndReturnTrueIfValueChanged()
     return changed;
 }
 
+QUntypedPropertyBinding::QUntypedPropertyBinding() = default;
+
 QUntypedPropertyBinding::QUntypedPropertyBinding(QUntypedPropertyBinding::BindingEvaluationFunction function,
                                                  const QPropertyBindingSourceLocation &location)
 {