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

[Michael Kruse via cfe-commits]

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Meinersbur wrote:

The filename (`foreach`) suggests testing range-based for-statement. There is not such loop in this test.

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