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

[Bill Wendling via cfe-commits]

bwendling wrote:

A quick look at `Decl` seems like it would require adding a `PrevInContextAndBits`, which would increase the size by `sizeof(Decl *)`. I might be able to massage a way to make it work, but it's probably not worth it.

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