Fix `RIEGELI_INTERNAL_INLINE_CONSTEXPR` in C++17 mode: refer to `::riegeli::internal::type_identity_t`, not `::riegeli::type_identity_t`.
Fix `RIEGELI_INTERNAL_INLINE_CONSTEXPR` in C++17 mode: refer to
`::riegeli::internal::type_identity_t`, not
`::riegeli::type_identity_t`.

Fixes https://github.com/google/riegeli/issues/12

PiperOrigin-RevId: 327200770

google/riegeli,google/riegeli,google/riegeli,google/riegeli