translate drop glue using MIR

Drop of arrays is now translated in trans::block in an ugly way that I
should clean up in a later PR, and does not handle panics in the middle
of an array drop, but this commit & PR are growing too big.

src/test/codegen-units/item-collection/trait-method-default-impl.rs

@@ -66,5 +66,3 @@ fn main() {
     //~ TRANS_ITEM fn trait_method_default_impl::SomeGenericTrait[0]::bar[0]<u32, i16, ()>
     0u32.bar(0i16, ());
 }
-
-//~ TRANS_ITEM drop-glue i8