[TASK] Non extension dirs in llXmlAutoFilename

Allow non extension dirs (e.g. fileadmin) in
\TYPO3\CMS\Core\Utility\GeneralUtility::llXmlAutoFilename()

Fixes: #34728
Releases: 6.2, 6.1, 4.7, 4.5
Change-Id: I073f42ec8b6d733246151df331201031187e1baf