[clang-format] Handle attributes in enum declaration.

Fixes https://github.com/llvm/llvm-project/issues/55457

Ensures that attributes in the enum declaration are interpreted
correctly, for instance:

```
enum class [[nodiscard]] E {
  a,
  b
};
```

Reviewed By: MyDeveloperDay, curdeius

Differential Revision: https://reviews.llvm.org/D125848