/usr/lib/mono/4.5/System.Xml.dll is in libmono-system-xml4.0-cil 3.2.8+dfsg-10.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/System.Xml/4.0.0.0__b77a5c561934e089/System.Xml.dll