[clang] [Clang][NFC] Improve testing for the flexible array member (PR #89462)

[Bill Wendling via cfe-commits]

bwendling wrote:

> Strictly speaking, I don't think this is NFC. Consider, for example:
> 
> ```
> struct S {
>   struct X { int array[1]; } x;
>   struct Y { int count; int array[] __attribute__((__counted_by__(count))); } y;
> };
> void f(void* __attribute__((pass_dynamic_object_size(0))));
> void g(struct S* s) { f(s->y.array); }
> ```

Hmm.. Yeah, this change makes it more accurate. I'll remove the tag.

https://github.com/llvm/llvm-project/pull/89462