[HLSL] Add support for unbounded resource arrays at global scope

Add support for unbounded resource array declaration at global scope. 

For example:
```
RWBuffer<float> D[] : register(u5);
RWBuffer<float> E[][4] : register(u0, space1);
RWBuffer<float> Out;

[numthreads(4,1,1)]
void main() {
  Out[0] = D[100][0] + E[1000][4][0];
}
```

Note that local declarations of unbounded arrays will not be supported by HLSL in Clang. See microsoft/hlsl-specs#141.