[llvm] [openmp] [OpenMP][Runtime] Handling crash with `OMP_TARGET_OFFLOAD=DISABLED` and invoking `omp_get_default_device()` (PR #171789)

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jprotze wrote:

I forgot to raise a more important issue with this patch: 
Instead of pretending that the ICV has a reasonable value when looking that it (using the OpenMP getter function), the ICV should always have a reasonable value when setting the ICV. Having an invalid value will create unexpected results in other places where the runtime directly uses the value from the ICV instead of calling the getter function.

https://github.com/llvm/llvm-project/pull/171789