[HLSL] Clang needs to support single subscript operators on matrix type

[farzonl]

Today clang will error with error: single subscript expressions are not allowed for matrix values.
But DXC does a vector load
https://hlsl.godbolt.org/z/PM46xW89v

RWStructuredBuffer<int> In;
RWStructuredBuffer<int> Out;
cbuffer Constants {
    uint row;
    uint col;
};
[numthreads(1, 1, 1)]
void CSMain(uint GI : SV_GroupIndex) {
    int2x3 M = int2x3(0, 1, 2, 
                      3, 4, 5);
    Out[0] = M[row].x;
}

The fix is likely that we need a new AST type to allow for single subscript.