[llvm] [OFFLOAD] Add olIterateCompatibleDevices API (PR #226441)

[Alex Duran via llvm-commits]

adurang wrote:

> Do we need this? I figured we could just export an 'iterateDevices' and a `isDeviceCompatbile` and leave the uesr to do it.

Yes, because otherwise you would init all Platftorms while doing iterateDevices. This first checks if a Platform is compatible, and only then initializes it, and traverses its devices (checking also if they're compatible).

https://github.com/llvm/llvm-project/pull/226441