[SYCL] deprecate ext_oneapi_get_last_event and ext_oneapi_set_externa…

[igchor]

…l_event

[EwanC]

FYI this extension is used as an implementation detail of async_malloc https://github.com/intel/llvm/blob/sycl/sycl/source/detail/async_alloc.cpp#L84

So if we're depreciating it, then we should update that implementation, or create an issue to do so and reference it in a comment next to that line of code.

[igchor]

FYI this extension is used as an implementation detail of async_malloc https://github.com/intel/llvm/blob/sycl/sycl/source/detail/async_alloc.cpp#L84

So if we're depreciating it, then we should update that implementation, or create an issue to do so and reference it in a comment next to that line of code.

That's a good point. I'll take a look at how we can modify the implementation