[Clang][OpenMP] Add `#pragma omp flatten` loop directive + `depth` clause (#206977)

Adds the OpenMP 6.1:  `Flatten Construct` + `depth Clause`.
Referred in Section 17.4 and Section 6.4.7 respectively in [Technical
Report 15](https://www.openmp.org/wp-content/uploads/openmp-TR15.pdf)

GitOrigin-RevId: fd016b7d24d3a2ddabef9e1ebfc702885e9bb7c1