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

[Michael Kruse via cfe-commits]

https://github.com/Meinersbur commented:

Consider updating https://github.com/llvm/llvm-project/blob/main/clang/docs/OpenMPSupport.md

Looks good so far just some smaller remarks from me

https://github.com/llvm/llvm-project/pull/206977