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

llvm-svn: 297083