Improve reflection types - add not-null properties to certain interfaces

[TahirAhmadov]

I asked this question previously about XmlDocument, and I have a similar suggestion now for things like FieldInfo.DeclaringType - that property can never be null, yet since it's inherited from MemberInfo, it's marked as nullable (because Type is also a MemberInfo and it can have null DeclaringType). I think we should add public new Type DeclaringType { get; } properties to FieldInfo, MethodInfo, etc.