Resolve remaining Xslt-related warnings (round 2) (#50211)

Resolve remaining Xslt-related warnings (round 2) (#50211)

* Resolve remaining Xslt-related warnings

* Addressing final linker warning and adding annotations to ref assembly

* Update src/libraries/System.Private.Xml/src/System/Xml/Xsl/IlGen/GenerateHelper.cs

Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
* Update src/libraries/System.Private.Xml/src/System/Xml/Xsl/Runtime/XmlExtensionFunction.cs

Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
* Addressing PR feedback

* kept -> preserved

* Address remaining feedback

Co-authored-by: Eric Erhardt <eric.erhardt@microsoft.com>