CL: Support pointers to private memory

This series updates nir_lower_explicit_io to support "private" variables, in shader/function temp storage, by using scratch intrinsics. It then maps these scratch intrinsics to DXIL versions which use integer indices rather than byte indices (with appropriate read-modify-write behavior). Lastly, it updates the DXIL backend to deal with private memory similarly to shared, by emitting a single alloca for the scratch space.

After all that's done, I deleted our previous function_temp handling.

/cc @daniels @bbrezillon @kusma @jekstrand @karolherbst