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

[Phoebe Wang via cfe-commits]

phoebewang wrote:

> I've revised it. Can you check it again? @phoebewang

I'm not sure the usage of the `isEmptyRecord`. Tagging @asb who modified the interface recently.

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