Add `--xml` flag to persist xml artifacts for testing purposes (#56238)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56238

Currently, all xml artifacts generated by doxygen are stored in the tmp dir and deleted at the end of the snapshot generation. For debugging reasons, this diff adds `--xml` flag to persist generated artifacts, so that they can be analyzed.

Changelog:
[Internal]

Reviewed By: cipolleschi

Differential Revision: D98289956

fbshipit-source-id: 73eb0185f7112389e589d3e8f8a2004efe325e8d