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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic approved this pull request.

I'm still fine with this.  And I think @rjmccall's concerns were addressed by the latest version.

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