[libc++][NFC] Replace typedefs with using declarations (#156009)

We've done quite a bit of refactoring recently in `<__tree>`. This patch
finishes up replacing typedefs with using declarations. As a
side-effect, this also adds some `_LIBCPP_NODEBUG` annotations, since
the clang-tidy check catches these now.

NOKEYCHECK=True
GitOrigin-RevId: 3a1298b943fe3beea60e6a6f2dff154620774aff
1 file changed