[llvm] [NFC][Offload] Clarify `olDestroyQueue` (PR #152132)

[Callum Fare via llvm-commits]

callumfare wrote:

> Actually implementing it will probably require something like `olEnqueueCallback` to enqueue a callback at the end of the queue to clean it up.

I don't think I follow why this is necessary, can't `olDestroyQueue` implicitly call `olSyncQueue`? Intuitively I would expect destroying a queue to block until the work is finished

https://github.com/llvm/llvm-project/pull/152132