Audit S.X.S.XmlSerializer for Ref.Emitting code

[MichalStrehovsky]

We should audit places in historic versions of XmlSerializer sources that had FEATURE_SERIALIZATION_UAPAOT defines and friends.

Those might need to be restored so that we don't try to do Ref.Emit crap at runtime.

#593

I didn't find the place where the defines were deleted, but the UAPAOT branch of corefx will have the "known good" version of the serializer compiling for UAPAOT.

[MichalStrehovsky]

Fixed in #600.