[OPENMP][CUDA]Use __syncthreads when compiled by nvcc and clang >= 9.0.
[OPENMP][CUDA]Use __syncthreads when compiled by nvcc and clang >= 9.0.

Summary:
The problems with __syncthreads() were fixed in clang >= 9.0 and the
original __syncthreads() can be used instead of the ptx instruction.

Reviewers: grokos

Subscribers: guansong, jdoerfert, openmp-commits, kkwli0, caomhin

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D63515

git-svn-id: f99161ee8ccfe2101cbe1bdda2220bce2ed25485@363807 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/openmp,llvm-mirror/openmp,llvm-mirror/openmp,llvm-mirror/openmp,llvm-mirror/openmp