Survey current `ConstantBuffer<T>` support in DXC and add tests to offload-test-suite

Document what can be done with `ConstantBuffer<T>` in DXC (local variables, arrays, ...).
Add a set of test cases for this to offload test suite.

```
struct S {
  int a;
  float4 b;
};

RWBuffer<float> Buf;

ConstantBuffer<S> CBS[2];

[numthreads(4,1,1)]
void main(int i : SV_GROUPID) {
    ConstantBuffer<S> cb = CBS[i % 2];
    Buf[0] = cb.b.x;
}
```
https://godbolt.org/z/M8TjExTnq