[mypyc] Preserve inherited attribute defaults under separate=True (#21547)

Fixes #21542

Under `separate=True`, when a subclass is recompiled while its parent is
loaded from mypy's incremental cache, parent default-attribute
assignments are silently dropped from the subclass's
`__mypyc_defaults_setup`. The first read of an inherited default-attr
then raises:

```
    AttributeError: attribute '<name>' of '<Parent>' undefined
```

`find_attr_initializers` walks `cdef.info.mro` and reads
`info.defn.defs.body` for `AssignmentStmt`s. `ClassDef.serialize`
(mypy/nodes.py) does not serialize `defs`, so a cache-loaded parent has
`defs = Block([])`; the MRO walk collects no parent assignments and the
subclass's emitted setup leaves inherited slots in the
undefined-sentinel state.

This PR implements the fix discussed in the linked issue.