Add AdditionalMember to set LangOptions for WorkGroupSize

[smanna12]

I sort of wonder whether we'd be doing a good thing by having an AdditionalMember like:

const Expr *getWorkGroupSize(const LangOptions &LO) const {
  return LO.OpenCL ? getXDim() : getZDim();
}

Perhaps not as part of this PR though -- this may be worth thinking about for when we correct the behavior of the one, two, and three arg forms of the attribute.

Originally posted by @AaronBallman in #3728 (comment)