[Bounds-Safety] Temporarily relax a `counted_by` attribute restriction on flexible array members

In 0ec3b972e58bcbcdc1bebe1696ea37f2931287c3 an additional restriction
was added when applying the `counted_by` attribute to flexible array
members in structs. The restriction prevented the element type being
a struct that itself had a flexible array member. E.g.:

```
struct has_unannotated_VLA {
  int count;
  char buffer[];
};

struct buffer_of_structs_with_unnannotated_vla {
  int count;
  struct has_unannotated_VLA Arr[] __counted_by(count);
};
```

In this example assuming the size of `Arr` is `sizeof(struct
has_unannotated_VLA)*count` (which is what the attribute says) is wrong
because it doesn't account for the size of
`has_unannotated_VLA::buffer`. This is why this kind of code construct
was treated as an error.

However, it turns out existing Linux kernel code used the attribute
on a flexible array member in this way
(https://github.com/llvm/llvm-project/pull/90786#issuecomment-2118416515).

To unbreak the build this restriction is downgraded to a warning with
the plan to make it an error again once the errornous use of the
attribute in the Linux kernel is resolved.