[clang-tidy] fix false positve for namespace with attrs in modernize-concat-nested-namespaces

Fixed https://github.com/llvm/llvm-project/issues/57530
Add pre check to avoid false positive for namespace with attributes like
```
namespace [[deprecated]] ns {}
```

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D147857