[llvm] [BPF] Allow struct pointee member btf generation with annotations (PR #141719)

[via llvm-commits]

eddyz87 wrote:

I noticed a single significant change in BTF generated for selftests:

```
-6.6K   verifier_bits_iter.bpf.o.btf
+69K    verifier_bits_iter.bpf.o.btf
```

Happens because of a `task_struct` as far as I understand.

>From logic pow, I think this code change is fine, however, I'm curious, would it be possible to use a single flag (e.g. `StopAtPointer`) instead of two flags (`SeenPointer`, `CheckPointer`)?

https://github.com/llvm/llvm-project/pull/141719