[Bug target/120241] bpf: problem with preserve_access_index in "nested" struct types

[dfaust at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120241

--- Comment #3 from David Faust <dfaust at gcc dot gnu.org> ---
The problem with the test is that he inner anonymous union does _not_ have
preserve-access-index attribute.
So it makes sense we stop there for CO-RE.

But that means that the CO-RE relocation we generate ends up resolving to a
union type, rather than the pointer type within the union.  The verifier
rejects the relocation as invalid because it must resolve to a pointer type.

So I think:
1. The test should be fixed to properly attribute the anonymous union.
2. We should try to detect this case in gcc and warn/error.