[clang] [AArch64] Match GCC behaviour for zero-size structs (PR #124760)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The likelihood of anyone actually tripping over this outside a synthetic testcase is very low; you need a combination of unusual constructs to hit this.  So I'm not really worried about documentation/ABI compat flags.

https://github.com/llvm/llvm-project/pull/124760