R1:a1561a4d3a07

Fix a fatal in Diviner when atoms extend ghosts

Summary:
Fixes T8547. I wasn't immediately able to reproduce this locally (although I didn't try too hard), but I think the issue is that when atoms extend ghosts (probably they are usually ghosts themselves?), we try to check the ghost language and fatal.

Instead, don't match ghosts when figuring out what an atom extends.

This could maybe be a little cleaner (match the ghosts, at lower priority, and show that they're ghosts?) but I'm not sure there's a…