[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
4 files changed