This change adds a new macro, ABSL_HARDENING_ASSERT_SLOW, which can be used for hardening assertions which have too great a performance impact to enable widely.  It also adds an additional supported value for ABSL_OPTION_HARDENED to only enable assertions which are not slow.  ABSL_OPTION_HARDENED=1 will continue to enable all hardening assertions, while ABSL_OPTION_HARDENED=2 will only enable assertions not marked as SLOW.

PiperOrigin-RevId: 668976425
Change-Id: I12cb209715d5b359c4093cc8c62e16794d7729b3