[clang-tools-extra] [clang-tidy] Add `readability-redundant-tag` check (PR #210007)

[Yanzuo Liu via cfe-commits]

zwuis wrote:

Please add more tests. E.g.

- Hidden in inner scope.

```cpp
struct S {};
namespace NS1 {
int S;
namespace NS2 {
using T = struct S;
}
}
```

- Involving using declarations.

```cpp
namespace NS {
struct S {};
}
using NS::S;
using T = struct S;
// `S` is not 'hidden' in this example.
// Just to tell using declarations should be considered.
```

https://github.com/llvm/llvm-project/pull/210007