[clang] fix: C++ empty record with align lead to va_list out of sync (PR #72197)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

The changes look reasonable to me, but I'd appreciate if @rjmccall or @efriedma-quic could make the final sign-off.

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