[X86_64] Fix empty field error in vaarg of C++. (#101639)

Such struct types:
```
struct {
  struct{} a;
  long long b;
};

stuct {
  struct{} a;
  double b;
};
```
For such structures, Lo is NoClass and Hi is Integer/SSE. And when this
structure argument is passed, the high part is passed at offset 8 in
memory. So we should do special handling for these types in
EmitVAArg.Fix https://github.com/llvm/llvm-project/issues/79790 and fix
https://github.com/llvm/llvm-project/issues/86371.

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