[llvm] [Offload][OpenMP] Accept `omp_initial_device` for runtime calls (PR #205290)

[Jan André Reuter via llvm-commits]

Thyre wrote:

> I don't think it would take much to have a single `bool isInitialDevice(int deviceNum)` helper defined and used across all those conditions. Seems like a much more future-proof change.

That's true, though I'm not sure where to best add the helper function (or potentially macro).
The function would be used in `interface.cpp`, `API.cpp` and `omptarget.cpp`.
Maybe in `InternalTypes.h`?

https://github.com/llvm/llvm-project/pull/205290