[clang] [Sema] Reject unqualified lookup of local nested class name (PR #192678)

[Yanzuo Liu via cfe-commits]

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zwuis wrote:

I meant, Clang already rejects

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

correctly. How it rejects? Can we use similar logic to handle

```cpp
void f() {
  struct A { struct B; };
  struct A::B {};
  B b;
}
```

?

https://github.com/llvm/llvm-project/pull/192678