[XLinq] create new instance of XmlSchemaInfo for each XObject from XmlReader.

[XLinq] create new instance of XmlSchemaInfo for each XObject from XmlReader.

In referencesource, XmlReader.SchemaInfo does not return different instances
for each node at its state but just returns the XmlReader itself (which
implements IXmlSchemaInfo). Hence, if we just set the corresponding
IXmlScemaInfo to each XObject, it results in inconsistent state.

This fixes two regressions in ExtensionsTest (XAttributeFailValidate and
XAttributeThrowExceptionValidate).