[Offload] Lazily initialize platforms in the Offloading API (#163272) Summary: The Offloading library wraps around the underlying plugins. The problem is that we currently initialize all plugins we find, even if they are not needed for the program. This is very expensive for trivial uses, as fully heterogenous usage is quite rare. In practice this means that you will always pay a 200 ms penalty for having CUDA installed. This patch changes the behavior to provide accessors into the plugins and devices that allows them to be initialized lazily. We use a once_flag, this should properly take a fast-path check while still blocking on concurrent use. Making full use of this will require a way to filter platforms more specifically. I'm thinking of what this would look like as an API. I'm thinking that we either have an extra iterate function that takes a callback on the platform, or we just provide a helper to find all the devices that can run a given image. Maybe both? Fixes: https://github.com/llvm/llvm-project/issues/159636
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