[clang] [X86_64] fix empty structure vaarg in c++ (PR #77907)

[Phoebe Wang via cfe-commits]

phoebewang wrote:

Checked both pr77036.cpp and pr77036.c get the same result, so looks like a right fix. But I want to wait @efriedma-quic to sign off.

https://github.com/llvm/llvm-project/pull/77907