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

[Amit Tiwari via cfe-commits]

loopacino wrote:

> Will it handle this test:
> 
> ```
> #pragma omp flatten
> for (int i = 0; i < -1; ++i)   // also: runtime n,m both negative
>   for (int j = 0; j < -1; ++j)
>     body(i, j);                // must execute 0 times; flatten executes it once
> ```
> 
> ?

clamped each trip count to the range max(0,N) before multiplying so -1 is 0 and hence _now_ runs zero times.

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