[clang-tidy] Fix `DefaultHungarianPrefix` for incomplete classes in `readability-identifier-naming` (#215734) Treat `DefaultHungarianPrefix` as a valid `readability-identifier-naming` option and check that a class has a definition before calling `isAbstract()` to compute its Hungarian prefix. Closes #215732 GitOrigin-RevId: d8dece650e84a1647ceaa035327fc3d8989accfd