[llvm] Use liboffload to initialize libomptarget plugins (PR #220260)

[Alex Duran via llvm-commits]

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adurang wrote:

This is not a matter of resource optimization, it's a matter of correctness:
* OMPT calls happens inside device initialization (happens inside olInit)
* OMPT calls need an UserID associated with each Device
* To compute the ID you need to know which device are *actually* going to be used based on the image (which can only be done after olinit)

I'm sure you see this impossible to achieve right now.

https://github.com/llvm/llvm-project/pull/220260