fix calling destructors on qualified structs

+ optimize by not calling TypeInfo.next again and again
+ remove type info argument from GC.setAttr/setBits (I thought that was already in the previous PR)

The optimizations make the druntime/benchmark a little faster than the version from master.
Unless it's possible to overload the postblit operation by qualifier, then I don't believe it should have an effect. I'm not near a computer I can use to test it currently though.