Add AdditionalMember to set LangOptions for WorkGroupSize

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 https://github.com/intel/llvm/pull/3728#discussion_r632719591_