[llvm] [llvm-mt] Use XmlDeleter to free xmlFreeDoc (PR #128472)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

> I am not sure what is wrong with CI, but I see the same on no-op patch #128563 So I assume my changes are unrelated.

CI was correct, I misread the log.

https://github.com/llvm/llvm-project/pull/128472