[HLSL] Invalid error reported for `int3` vector with `packoffset`

[hekota]

The following code reports error: packoffset cannot cross register boundary in Clang while DXC compiles it fine.

cbuffer CB  {
  int2 c : packoffset(c1.y);
};

[numthreads(4,1,1)]
void main() {}

https://godbolt.org/z/4as9ebned

The problem is that in Clang the width and alignment of the int3 type is 128 bits because it must be a power of 2. HLSL seems to align vectors based on the width of a single element.

[spall]

It looks like this issue is related to issue #104503.

[damyanp]

Blocked on #123968

[damyanp]

@bogner to check how this interacts with the cbuffer changes he's working on.