[clang] fix: compatible C++ empty record with align UB with gcc (PR #72197)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Good to see this works without any unexpected side-effects.

Please also add a test for 32-byte alignment (since that generates different code)

https://github.com/llvm/llvm-project/pull/72197