[xla:gpu] Track loop iteration counter of a WhileThunk in thread local variable
[xla:gpu] Track loop iteration counter of a WhileThunk in thread local variable

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/13056 from openxla:aportnoy/ffi-docs-fix-result-syntax 7940a1a02a0f93736a88406958edf62488bdbe19
