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

[via cfe-commits]

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arrowten wrote:

@zwuis This change fixes the lookup behaviour for local classes inside function scope too. So the code
```
void f() {
  struct A { struct B; };
  struct A::B {};
  B b;
}
```
is getting rejected. 

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