swift

fa578dfb - Simplify @objc actor back-deploy IR and switch to `objc_getRequiredClass`.

Simplify @objc actor back-deploy IR and switch to `objc_getRequiredClass`. `objc_getRequiredClass` will produce a fatal error if the class isn't found, which will prevent a malformed program using back-deployed @objc actor from launching. Also eliminate the spurious `objc_opt_self` call, which is unneeded given that we're realizing the metadata. Thanks to Mike Ash for the review.