[llvm] [Offload] Replace "EventOut" parameters with `olCreateEvent` (PR #150217)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

Under the hood, this is how it works anyway; all of the enqueue methods just call `makeEvent()` at the end. I was looking at adding an output event for `olWaitEvents()` (which is required for the UR adapter), but having every function that touches a queue accept an extra parameter just in case you want to capture an event feels a bit cumbersome.

https://github.com/llvm/llvm-project/pull/150217