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

[Yanzuo Liu via cfe-commits]

================

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zwuis wrote:

It seems that my question is still not clear. 🤦 You added these `if` to handle

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

, but Clang already handles

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

(not in function body) correctly without these if. Could you please tell, in our implementation, how/why Clang handled (before this PR) them differently?

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