Reference assemblies lack annotations

[dgrunwald]

Even on .NET Core 3.1, not all system libraries are annotated.

For example, the System.Linq methods are lacking annotations.
This means a call like collection.FirstOrDefault() will:

• incorrectly allow nullable collections
• incorrectly allow the return value to be non-nullable

This can cause significantly wrong annotations being inferred (and then accepted by the compiler without warning).

Maybe we should somehow include the .NET 5 annotations with the inference tool, so that it can produce useful results on projects targeting .NET Core 3 or even .NET Framework 4.x?