[flang][OpenMP] Fix firstprivate with barrier

This patch fixes the unintentional data race in firstprivate
implementation. There is a Read-Write race when one thread tries
to copy the value inside the omp.parallel region while other
thread modifies it from inside the region (using pointers or
some other form of indirect access).

For detailed discussion please refer to [[ https://discourse.llvm.org/t/issues-with-the-current-implementation-of-privatization-in-openmp-with-fortran/62335 | discourse ]].

Reviewed By: kiranchandramohan, peixin, NimishMishra

Differential Revision: https://reviews.llvm.org/D125689