[OpenMP] Put old APIs back and added new _async series for backward compatibility

Summary: According to comments on bi-weekly meeting, this patch put back old APIs and added new `_async` series

Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: yaxunl, guansong, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D77822