Make borrowck's notion of scopes consistent with trans's notion of scope...

r? @nikomatsakis 

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.