[ReflectionBasedXmlSerializer] To Support SpecialMapping Types Having DerivedMappings

[shmao]

See https://github.com/dotnet/corefx/blob/32c4013a5d750c1c6a5b240d37a689991a479097/src/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationReader.cs#L990

[dotnet-policy-service]

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