Fix existential specializer for unowned ownership

[meg-gupta]

init_existential_ref cannot forward operands of unowned ownership. Create an owned value when we have such args.

[meg-gupta]

@swift-ci test

[meg-gupta]

I don't know why exactly it is illegal for init_existential_ref to have operands with unowned ownership. We allow certain casts like unchecked_ref_cast to have operand with unowned ownership. This PR fixes the verification error by creating owned values.

[eeckstein]

lgtm

[atrick]

I don't know why exactly it is illegal for init_existential_ref to have operands with unowned ownership. We allow certain casts like unchecked_ref_cast to have operand with unowned ownership. This PR fixes the verification error by creating owned values.

unowned values need to be shielded from all the normal OSSA transformations. They can't be transformed safely. The general rule should be that you can never get an owned value from unowned without a copy. We might have some very special casts that unsafely convert ownership; we need to make sure not to optimize those.