[libc++] Partially revert #208330 (#209928)

We found out that the new mechanism for detecting overriden functions
does not support Arm Pointer Authentication (PAuth). Addressing this
limitation is going to require changes to Clang. In the meantime, we
switched back to the old mechanism when PAuth is enabled.

GitOrigin-RevId: db7356dde8312a86e79ec0d7b7f844878a1a743c