[clang] [Clang][Sema] Fix crash in containsUnexpandedParameterPacks (PR #182484)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt commented:

Ok, so the root issue I think is related to k&r function decls (which is why it doesn't work in c++), to make it slightly clearer, this repros the issue:

```c
void g(h(z)...);
```

This appears to result in us diagnosing the ellipsis error after parsing the param name (k&r, hooray), but we haven't yet parsed the rest of the decl when emitting the ellipsis diagnostic.

https://github.com/llvm/llvm-project/pull/182484