HLSL: UAV counters always in descriptor set 0

It looks like glslang always puts the UAV counters in ```DescriptorSet 0```. @danginsburg and I chatted about this briefly and were wondering if they should remain in the same descriptor set as their respective UAV buffers.

Sample shader compiled with ```--amb```:
```C
struct Data {
float4 v;
};
ConsumeStructuredBuffer buf_in : register(u0, space1);
AppendStructuredBuffer buf_out : register(u2, space3);

float4 main(float4 sv_pos : SV_POSITION) : SV_TARGET
{
Data val = buf_in.Consume();
buf_out.Append(val);
return sv_pos;
}
````

...produces...
````
Decorate 18(buf_in) BufferBlock
Decorate 20(buf_in) DescriptorSet 1
Decorate 20(buf_in) Binding 0
MemberDecorate 23(buf_in@count) 0 Offset 0
Decorate 23(buf_in@count) BufferBlock
Decorate 25(buf_in@count) DescriptorSet 0
Decorate 25(buf_in@count) Binding 0
Decorate 39(buf_out) DescriptorSet 3
Decorate 39(buf_out) Binding 2
Decorate 40(buf_out@count) DescriptorSet 0
Decorate 40(buf_out@count) Binding 1
````

```buf_in@count``` and ```buf_out@count``` are both in ```DescriptorSet 0``` while ```buf_in``` and ```buf_out``` are in ```DescriptorSet 2``` and ```DescriptorSet 3```, respectively.

Start by compiling the provided HLSL sample with the mentioned --amb option and inspect the generated descriptor-set decorations for buf_in@count and buf_out@count. Done means establishing and implementing the correct descriptor-set relationship between each UAV counter and its corresponding UAV buffer, then confirming the sample output.

Written by the indexing model from the issue text.