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

[Longsheng Mou via cfe-commits]

CoTinker wrote:

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

It seems that `struct S14{}` is empty record, but  `struct S14 { union{}a;}` is not.

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