Avoid scanning all references for DynamicInterfaceCastableImplementation

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.