foreach: fix case where iterators are not on the stack.

foreach: fix case where iterators are not on the stack.

The foreach garbage collection assumed that iterators were all on the
stack, but they could be on the heap or a global (or static) variable.
We can prevent the heap case by tricky use of macros to complain on
any iterator which isn't a single token, but we can't prevent
globals/statics.

So, if an iterator already seems to be "off" the stack, mark it as
such and simply never free it.