[clang] [clang] Add another test case for CWG78 (PR #93708)

[via cfe-commits]

cor3ntin wrote:

We should test something like that

```cpp

struct S {
S();
};


S s;
const S s
```

Here.
The IR should should that s is zero initialized (before calling S::S())






https://github.com/llvm/llvm-project/pull/93708