Change Microsoft.XmlSerializer.Generator to not use private reflection on XElement constructor

[eerhardt]

See the discussion in #35725, specifically this comment: #35725 (comment).

We should change the C# generation to special case XElement (just like we did for the IL generation case), and have it generate new XElement("default") instead of calling Activator.CreateInstance. Using private reflection can easily be broken in the future, so we shouldn't rely on it.

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[danmoseley]

Per https://github.com/dotnet/runtime/blob/master/docs/area-owners.md this is owned by @HongGit @StephenMolloy