[clang] [Clang][Sema] Apply CWG400 'Using-declarations and the "struct hack"' to C++98 mode (PR #143492)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

I wrote wrong test case in PR description. I apologize. The following is being fixed:

```cpp
struct A {};
struct B : A {
  using B::A;
};
```

PR description was updated. I'm away from my computer and I will add this code to test files next Monday.

https://github.com/llvm/llvm-project/pull/143492