[OpenACC] Partial Reduction recipe Lowering (#155635)
This patch implements basic reduction recipe lowering, plus adds a bunch
of tests for it that should be meaningful later. At the moment, all this
does is ensure that we get the init 'alloca' set right (the actual
initializer isn't done correctly yet, and will be in a followup), an
empty combiner (though the type of certain operations probably has to be
different as well, when we get to those), and a full-destruction, as we
already have the infrastructure for it.
