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

[Kees Cook via cfe-commits]

kees wrote:

Does this still work for cases where there are multiple flexible arrays? e.g.

```
struct weird_protocol {
    unsigned int cmd_type;
    unsigned int data_len;
    union {
        struct cmd_one one[];
        struct cmd_two two[];
        struct cmd_three three[];
        unsigned char raw_bytes[];
    };
};
```

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