PublicApiAnalyzer should add info about members made `[Obsolete]`

[JohanLarsson]

[Evangelink]

Hi @JohanLarsson,

Are you expecting only some information that some member is now becoming obsolete? Or are you also thinking about some more advanced scenario with some kind of link to another member?

[JohanLarsson]

I was thinking simple to start with, could be nice to see at what versions things were made obsolete when diffing PublicAPI.Shipped.txt. Maybe just suffix the line with [Obsolete]?