Code Modernization: Use correct property in IXR_Message::tag_open().
The IXR_Message class declares a property _currentTag, which is never assigned or used. It does assign to currentTag instead, which outside of that one assignment is never used either.
Since there are various other underscore-prefixed properties declared on the class, including one named _currentTagContents which is used in several places, it appears that the declared property is correct and the assignment is a typo.
This commit resolves a notice on PHP 8.2:
Deprecated: Creation of dynamic property IXR_Message::$currentTag is deprecated
Follow-up to [1346].
Props bjorsch, kraftbj, jrf, mukesh27, SergeyBiryukov.
See #56790.