[libcxxabi] Pulled guard byte code out of GuardObject

Right now, GuardObject is in charge of both reading and writing to the guard
byte, and co-ordinating with the InitByte... classes. In order to improve
separation of concerns, create a separate class responsible for managing the
guard byte and use that inside GuardObject.

This is the 3rd of 5 changes to overhaul cxa_guard.
See D108343 for what the final result will be.

Depends on D110088

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D115367