remove type descriptors from proc and @T

This also drops support for the managed pointer POISON_ON_FREE feature
as it's not worth adding back the support for it. After a snapshot, the
leftovers can be removed.
allow generating drop glue without the TyDesc

Reflection is now the only user of type descriptors. Uses of drop glue
no longer cause a type descriptor to be generated.
rm out-of-date comment from std::unstable::raw
auto merge of #12059 : thestinger/rust/glue, r=pcwalton

A follow-up from the work I started with 383e3fd13b99827b5dbb107da7433bd0a70dea80.