[flang][OpenMP] Extend `do concurrent` mapping to device.

[ergawy]

For simple loops, we can now choose to map do concurrent to either the host (i.e. omp parallel do) or the device (i.e. omp target teams distribute parallel do).

In order to use this from flang-new, you can pass: -fopenmp -fdo-concurrent-parallel=[host|device|none]; where none will disable the do concurrent mapping altogether.