Fix bug in virtual annotation logic for RequiresUnreferencedCode

[sbomer]

The following virtual methods are correctly annotated and do not warn:

class Base {
  virtual void M() {}
}

[RequiresUnreferencedCode("Derived")]
class Derived : Base {
  override void M() {}
}

However, if the method also happens to have DynamicallyAccessedMembers annotations, there's a bug in the logic that causes this to produce warnings in illink. This change fixes the bug.

Hit this while experimenting with ComponentModel annotations.

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[jtschuster]

What exactly are the semantics of RUC on a type? Is it like putting it on the .ctor and static methods, or all methods, static and instance? LGTM if it's the former, but if it's the latter then the existing behavior doesn't seem right.

[sbomer]

It's a bit of both - it will silence warnings inside of all methods, static and instance, but it will only produce warnings for callsites to static methods and constructors. Since we protect constructors, there's no need to warn for calls to virtual methods.

By the same reasoning, we don't need to warn for the example above because we've already prevented calls to constructors of the type.

[sbomer]

Failure is known: #100249. I've updated the error text to be slightly looser, but build analysis hasn't matched it with this build yet.

[sbomer]

Ah, it figured it out after a rerun.