[OpenMP] Introduce aligned synchronization into the new device RT

We will later use the fact that a barrier is aligned to reason about
thread divergence. For now we introduce the assumption and some more
documentation.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D112153

GitOrigin-RevId: b16aadf0a79a2796c9ac68d1bfc8853ccf25c459
5 files changed