Postpone marking of instance fields on ref types

[mateoatr]

This was a TODO in TypeDefinitionNode.cs

[MichalStrehovsky]

I just remembered there was a problem when I introduced this optimization in IL Linker. CoreLib does an Unsafe.As<SomeNeverAllocatedType>() cast and then accesses a field on the result of the cast but we stripped the unused fields from it, making things misaligned.

The fix was to also keep all the instance fields if any instance field on an Sequential/Explicit reference type is kept.

Maybe a good fix for that would be to:

• Introduce a new InstanceFieldLayoutNode that will be similar to ConstructedTypeNode except: No conditional static dependencies on this one, and the static dependencies are all the instance fields + the InstanceFieldLayoutNode of the base type.
• ConstructedTypeNode of reference types with explicit/sequential layout depends on the InstanceFieldLayoutNode of the type.
• FieldDefinitionNode of a reference type with explicit/sequential layout depends on the InstanceFieldLayoutNode of the owning type.