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

[Phoebe Wang via cfe-commits]

phoebewang wrote:

I checked it locally, the patch doesn't fix the reported problem:
```
$ clang pr77036.cpp && ./a.out
-nan
Fail
```

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