[OPENMP] Remove explicit call for implicit barrier

[OPENMP] Remove explicit call for implicit barrier
#pragma omp parallel needs an implicit barrier that is currently done by an explicit call to __kmpc_barrier. However, the runtime already ensures a barrier in __kmpc_fork_call which currently leads to two barriers per region per thread.
Differential Revision: http://reviews.llvm.org/D15561

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@255992 91177308-0d34-0410-b5e6-96231b3b80d8