swift234

Reflection: Add instance-specific layout entry point, and do some refactoring

Closure context layout will depend on the instance itself as well
as the isa pointer, because instead of instantiating metadata for
closures that capture generic parameters, we store the substitutions
inside the context itself.

For classes, this entry point just reads the isa pointer, applies
the isa mask and proceeds down the metadata path.

For now, the only the latter is hooked up.