`clang-format`sorts or groups namespaces in using declarations in a case-insensitive way

`clang-format` seems to treat the namespaces in a series of using declarations in a case insensitive way.

For example, this code:

```cpp
namespace testing {
class A {};
class C {};
class E {};
}

namespace Testing {
class B {};
class D {};
}

using Testing::B;
using Testing::D;
using testing::A;
using testing::C;
using testing::E;
```

Is reformatted to this:

```cpp
namespace testing {
class A {};
class C {};
class E {};
} // namespace testing

namespace Testing {
class B {};
class D {};
} // namespace Testing

using testing::A;
using Testing::B;
using testing::C;
using Testing::D;
using testing::E;
```

Which I think is both surprising and makes reading more difficult.

It's not just a case insensitive sort though -- if you prefix `testing` with `::`, you get the same order from `clang-format`, interleaving `::testing::...` and `Testing::...` entries.

Start by reproducing the reported ordering with the clang-format example and inspect the clang-format code that sorts using declarations. Add a regression test for namespaces whose names differ only by case, then verify that formatting no longer interleaves them unexpectedly.

Written by the indexing model from the issue text.