[SYCL] Fix discarded enqueue function event markings (#16223)
This commit fixes an issue where memory operations enqueued through the
enqueue free functions would not correctly mark the resulting events as
discarded, breaking in-order barrier assumptions.
Fixes https://github.com/intel/llvm/issues/15606.
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Co-authored-by: Andrey Alekseenko <al42and@gmail.com>
Signed-off-by: Larsen, Steffen <steffen.larsen@intel.com>
