Support OpenCL events in NIR and VTN
This is necessary to land before we can consume an updated libclc that includes the async copy functions. Otherwise the libclc shaders fail to translate. Similarly it'd be necessary for Clover if it wants to use a libclc build that includes async copy functions.
This is the first two commits from !112 (merged).
/cc @jekstrand @karolherbst @bbrezillon @daniels