[HLSL] Support RWBuffer loads and stores

[bogner]

This issue tracks support for basic RWBuffer support such that we can read and write from a simple buffer:

RWBuffer<int> In;
RWBuffer<int> Out;

[numthreads(8,1,1)]
void main(uint GI : SV_GroupIndex) {
  Out[GI] = In[GI];
}

[damyanp]

This has been superceded by llvm/wg-hlsl#7