For loops leak resources

[pczarn]

for (_, leaked) in Some(((), box 0u)).move_iter() {  }

// LEAK SUMMARY:
// definitely lost: 8 bytes in 1 blocks

cc @pcwalton

[bkoropoff]

Getting my feet wet in the rustc code a bit, pay no mind to my drive-by observations:

• This was introduced in caa564b
• Using a tuple is key to triggering the bug. Using a simple identifier as the for loop binding does not trigger it.
• This seems to be due to the special case for simple identifiers in store_for_loop_binding. Commenting out this special case results in a leak in all cases.

[bkoropoff]

The pop_custom_cleanup_scope on this line looks like it should be a pop_and_trans_custom_cleanup_scope. This fixes it for me. I'll submit a PR after running a full make check.