[clang-format] Fix wrong indentation of namespace identifiers after a concept declaration.

Before this patch, the code:
```
template <class T>
concept a_concept = X<>;
namespace B {
struct b_struct {};
} // namespace B
```
with config:
```
NamespaceIndentation: None
```

was wrongly indented inside namespace B, giving:
```
template <class T>
concept a_concept = X<>;
namespace B {
  struct b_struct {};
} // namespace B
```

Fixes https://github.com/llvm/llvm-project/issues/50645

Reviewed By: MyDeveloperDay, owenpan

Differential Revision: https://reviews.llvm.org/D116008