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

[Michael Kruse via llvm-commits]

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Meinersbur wrote:

Other tests use this to override the OpenMP version:
```
// RUN: %libomp-cxx-compile -fopenmp-version=61  && %libomp-run
```

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