This eliminates an ICE in trans where the scope for a particular
borrow was a statement ID, but the code in trans that does cleanups
wasn't finding the block with that scope. As per #3860
preserve looks at a node ID to see if it's for a statement -- if it
is, it uses the enclosing scope instead when updating the map that
trans looks at later.
I added a comment noting that this is not the best fix (since it may
cause boxes to be frozen for longer than necessary) and referring
to #3511.
r=nmatsakis
