[OpenMP][libomptarget] Flush intermediate values during team reduction

Summary: Ensure intermediate values of a team reduction are flushed to memory.

Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: guansong, jfb, openmp-commits

Differential Revision: https://reviews.llvm.org/D55219

llvm-svn: 348148