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.
There is only one xpostblit member in TypeInfo_Struct, so it cannot be overloaded right now (though I think Kenji made a proposal to allow qualifiers for them). I updated the postblit test to also check const structs.
https://issues.dlang.org/show_bug.cgi?id=8950 reports a similar problem, but this is in rt.assignarray.d or the compiler.