Avoid scanning all references for DynamicInterfaceCastableImplementation

[sbomer]

We currently scan all references for interfaces with DynamicInterfaceCastableImplementationAttribute, and keep any that implement marked interfaces. This attribute was designed to make the IDynamicCastableInterface feature linker-friendly, by explicitly annotating types that can serve as interface implementations for dynamic casts.

Depending on the expected behavior of GetInterfaceImplementation, we may be able to avoid this scan. There are already cases where we would not mark the attributed implementation (when GetInterfaceImplementation does non-analyzable reflection to get the implementing type's assembly, and that assembly isn't statically referenced).

However, we currently still scan referenced assemblies and keep such types (even if they are only used by non-analyzable reflection). Would it be safe to remove the attributed interfaces in this case? The linker already issues a warning for this.

[MichalStrehovsky]

Yes, I think that was the gist of how I was thinking to implement it in #1244 - "If an interface with DynamicInterfaceCastableImplementationAttribute is marked, do the same thing as if it was implemented by an instantiated type."

I didn't look at how the handling was implemented, but if we keep all interfaces marked as DynamicInterfaceCastableImplementationAttribute just because they require a marked interface, that seems more than what's strictly needed.

We should be able to postpone checking for DynamicInterfaceCastableImplementationAttribute until the interface is marked.