CollectedComments data in AstComment is invalidated

Dom crashes if you visit AstComments field of Qmlfile. The root cause is invalidated CommentedElement data of the AstComment class. When the visitor is visiting CommentedElement::iterateDirectSubpaths, the visited element's m_preComment and m_postComment are invalid if the current owner type is AstComments.

Workaround is to get the valid m_preComment and m_postComment from AstComment by explicit casting, see the fix for QTBUG-123547.

Investigate why CommentedElement is invalidated, when you revert the fix for QTBUG-123547.