black box flattening

[d-m-bailey]

I can't think of a use case where we'd want black boxes to be flattened out of existence.

However, in the pre-compare name check, if a black box doesn't exist in one of the circuits, the other one will be flattened.

Flattening unmatched subcell sky130_fd_sc_hvl__fill_2 in circuit xres_buf (1)(2 instances)
Flattening unmatched subcell sky130_fd_sc_hvl__fill_1 in circuit xres_buf (1)(5 instances)

Will submit a test case if necessary.

[RTimothyEdwards]

I started writing an answer to that one five different times. I think I understand the issue and why it fails. But I am uncertain enough that I would really like to see the test case.