[clang] [BPF] Support preserve_access_index attribute for enum (PR #232498)

[Andrii Nakryiko via cfe-commits]

anakryiko wrote:

@yonghong-song can we add some way to detect support for this without having to hard-code clang version? so instead of 

```
#if defined(__clang__) && __clang_major__ >= 24
```

we can have a bit more meaningful and backport-friendly

```
#if __bpf_relocatable_enum_attr
```

(I didn't think about good name, please don't take it verbatim)

https://github.com/llvm/llvm-project/pull/232498