[Mirror] Handle fields of thin metatypes.
Fields containing metatypes with no possible subtypes are thin i.e. they have no storage. There is only one possible value they can have: the corresponding type. Mirror attempted to copy the nonexistent field value from the nonexistent storage, producing garbage. Instead, special-case thin metatypes and copy the instance type out of the metatype metadata rather than trying to copy it from the field.
rdar://108280543
