clang-format: Improve formatting of annotated variables.

Before:
  bool aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa GUARDED_BY(
      aaaaaaaaaaaa) = aaaaaaaa::aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;

After:
  bool aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa GUARDED_BY(aaaaaaaaaaaa) =
      aaaaaaaa::aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;