Add build option LIBOMP_OMP_VERSION=50, 5.0 headers, and add the year/month associated with OpenMP 5.0 in relevant source locations. Also, remove the deprecated LIBOMP_OMP_VERSION=41 option. Patch by Olga Malysheva Differential Revision: https://reviews.llvm.org/D30450 git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@297083 91177308-0d34-0410-b5e6-96231b3b80d8