[clang-format] Handle attributes in enum declaration.

[tchatow]

Summary

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
};

[MyDeveloperDay]

This looks good to me, but let the others chime in..

[curdeius]

Could you please add test cases with non-empty enums both with and without comments please like in the bug report?

[tchatow]

Added tests for non-empty enums

[curdeius]

LGTM. Thanks a lot!
Do you have commit rights or need some help landing this?

[tchatow]

No, I'll need someone to commit it on my behalf.