QDBusIntrospection: make DiagnosticsReporter safe to use

Add a virtual destructor to this polymorphic class and disable copying
(requires to bring the default ctor back manually, and to export the
class nested in an exported class).

Amends 248d2103b5ef8f9cf8c1189cb67d78e1b6e741b7.

Change-Id: I9008e4ecebca34feac6ae92fa026f2673b652ba9
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit 02d7c8c069b93f7ddc1ccba462d8ff9ec3c1d806)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/dbus/qdbusintrospection.cpp

@@ -21,6 +21,9 @@ QT_BEGIN_NAMESPACE
     But they may prove useful if the XML data was obtained through other means (like parsing a file).
 */
 
+QDBusIntrospection::DiagnosticsReporter::~DiagnosticsReporter()
+    = default;
+
 /*!
     \class QDBusIntrospection::Argument
     \inmodule QtDBus

src/dbus/qdbusintrospection_p.h

@@ -58,9 +58,12 @@ public:
         qint64 columnNumber = 0;
     };
 
-    class DiagnosticsReporter
+    class Q_DBUS_EXPORT DiagnosticsReporter
     {
+        Q_DISABLE_COPY_MOVE(DiagnosticsReporter)
     public:
+        DiagnosticsReporter() = default;
+        virtual ~DiagnosticsReporter();
         virtual void warning(const SourceLocation &location, const char *msg, ...)
                 Q_ATTRIBUTE_FORMAT_PRINTF(3, 4) = 0;
         virtual void error(const SourceLocation &location, const char *msg, ...)