[libc++] Provide an observe semantic in the hardening mode

[ldionne]

Many people would benefit from having a way to turn on hardening without crashing their application when they violate a precondition. Instead, they would want to log the failure and continue. That way, they can enable hardening and gradually fix issues that come up in production, and eventually flip the switch without endangering their stability in production.

This is akin to the observe semantic in Contracts, so we need something like that eventually anyway.