[libcxxabi] Added convenience classes to cxa_guard

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

Depends on D115368

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D115369

GitOrigin-RevId: f011a53c148a8095e0bb63a92a0a640db16d1d54
2 files changed