Nullable annotations for System.Runtime.Serialization.Xml and System.Runtime.Serializ...

Nullable annotations for System.Runtime.Serialization.Xml and System.Runtime.Serialization.Json (#41476)

* Initial nullable annotations of System.Private.DataContractSerialization

Contributes to #2339

* Mark DataMember.Name as non-nullable.

* Fix a few simple nullable compile errors.

* Assert attributes is non-null in XmlObjectSerializerReadContext

* Ensure XmlObjectSerializerContext.serializer is never null

* Fix a few simple nullable errors

* Remove any checks that DataMember.MemberInfo can be null.

* Mark EnumDataContract.Members as non-nullable.

Fix nullable errors in SchemaExporter.

* Correctly annotate CollectionDataContract.IsCollectionOrTryCreate.

* Assert DataContractResolver is non-null.

* Suppress https://github.com/dotnet/runtime/issues/41465

* Update System.Runtime.Serialization.Json ref source for nullable annotations.

* Update System.Runtime.Serializaiton.Xml ref source for nullable annotations.

* Update for Xml.ReaderWriter nullable annotations

* Work around compiler issue.

* Fix test failure.

Reference compiler issue in TODO comment.

* Revert nullable suppression now that XmlSchemaAppInfo.Markup is annotated correctly.

* Fix build for latest annotations in master.

* PR feedback round 1

* Address PR feedback round 2