[OpenMP] Add ident_t flags for compiler OpenMP version

This patch adds the mask and ident_t function to get the
openmp version. It also adds logic to force monotonic:dynamic
behavior when OpenMP version less than 5.0.

The OpenMP version is stored in the format:
major*10+minor e.g., OpenMP 5.0 = 50

Differential Revision: https://reviews.llvm.org/D90632