swift234

IRGen: Fix broken destructors for heap arrays.

There were two problems here:

- We weren't storing the length of the array in the heap object, so the destructor never attempted to destroy any elements, and
- In the destructor loop, we were iterating backward but comparing the loop pointer against 'end', so if we actually initialized the length, the destructor would run forever destroying elements until it crashed on an invalid object.

Fixes <rdar://problem/16536439>.

Swift SVN r16138