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

[via cfe-commits]

cor3ntin wrote:

Hum, so with some help, I've realized that zero-initialization, and default-initialization _both_ occur, in that order (and the later is not valid) 

https://eel.is/c++draft/dcl.init#general-12
https://eel.is/c++draft/dcl.init#general-8.sentence-2

```cpp
int foo; // ok, foo == 0
const int bar; // ko
```

This a bit of an odd-behavior, but given that, I think we can mark the issue as implemented

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