ICE 981117 with new ABI

[Martin von Loewis]

The following code produces the ICE in finish_struct_1, as fields
is the type_dcl, and TYPE_FIELDS is a field_dcl:

struct input_iterator_tag {};
struct forward_iterator_tag : public input_iterator_tag {};

The compiler is egcs-2.92.23 19981127, on i486-pc-linux-gnu, with a
number of changes:

- squangling activated
- new_abi activated
- honor_std activated
- pending unicode change integrated
- pending dtor-ordering change integrated

I suspect the new_abi flag is causing this fault.

Martin