Make Event::PollForStatus a virtual method, and override it where necessary.
Make Event::PollForStatus a virtual method, and override it where necessary.

This enables Event to be a completely virtual base class, and removes it from circular dependencies.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/12942 from openxla:fix_test_cudnn c438b08ea7240c23ae98bc8dcf4ef45fa6d2e89c
