Legacy anon decls now have three uses:
* Type owner decls
* Function owner decls
* `@export` and `@extern`

Therefore, there are no longer any cases where we wish to explicitly
omit legacy anon decls from the binary. This means we can remove the
concept of an "alive" vs "dead" `Decl`, which also allows us to remove
the separate `anon_work_queue` in `Compilation`.