[.NET] Missing linker optimizations

[sbomer]

The .NET 6 linker no longer runs TypeMapStep for assemblies up-front. Instead the linker discovers base types on-demand (and the known overrides at any given point may be incomplete).

CoreTypeMapStep is not part of the .NET 6 macios linker pipeline, so some optimizations are not enabled:

• IsNSObject checks are not cached
  • This used to be cached via HashSet, where anything not in the set is considered "false". In the new world we can't assume that all types have been seen, so this would have to become Dictionary<TypeDefinition, bool> or similar using the linker custom annotations
• IsDirectBinding checks are not optimized (since they rely on information normally cached by CoreTypeMapStep)
  • Unless we scan some assemblies up-front, we won't know which wrapper types have derived types, so we'll probably need to keep scanning at least some assemblies before MarkStep. The pre-scan could be limited to assemblies which reference the product assembly.
  • As part of this, IsCIFilter also used to be cached. This could be done on type discovery with the new linker.

edit: another missed opportunity is that CollectUnmarkedMembersSubStep runs before MarkStep so it will effectively track all members. Ideally this step would run after Mark.

[spouliot]

CoreTypeMapStep also control the possible removal of the dynamic registrar from the final app. I'm moving this out of CoreTypeMapStep since it has no real interaction with the base type (or other code / optimization).

However it does, like other parts, assume all potential assemblies are known/loaded to make the decision (to keep or not the dynamic registrar). That decision does influence the optimization done later (before at at marking time).