[libcxxabi] Re-organized inheritance structure to remove CRTP in cxa_guard

Currently, the `InitByte...` classes inherit from `GuardObject` so they can
access the `base_address`, `init_byte_address` and `thread_id_address`. Then,
since `GuardObject` needs to call `acquire`/`release`/`abort_init_byte`, it uses
the curiously recurring template pattern (CRTP). This is rather messy.

Instead, we'll have `GuardObject` contain an instance of `InitByte`, and pass it
the addresses it needs in the constructor. `GuardObject` doesn't need the
addresses anyways, so it makes more sense for `InitByte` to keep them instead of
`GuardObject`. Then, `GuardObject` can call `acquire`/`release`/`abort` as one
of `InitByte`'s member functions.

Organizing things this way not only gets rid of the use of the CRTP, but also
improves separation of concerns a bit since the `InitByte` classes are no longer
indirectly responsible for things because of their inheritance from
`GuardObject`. This means we no longer have strange things like calling
`InitByteFutex.cxa_guard_acquire`, instead we call
`GuardObject<InitByteFutex>.cxa_guard_acquire`.

This is the 4th of 5 changes to overhaul cxa_guard.
See D108343 for what the final result will be.

Depends on D115367

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D115368