BDA struct members are not correctly reporting USED/UNUSED, are always reporting UNUSED

[danginsburg]

Creating tracking issue from discussion with @spencer-lunarg

 Slang uses OpPtrAccessChain instead of OpAccessChain that GLSL/HLSL does and this is why a pointer is still marked as "UNUSED"

We're seeing that shaders with struct BDA pointers are not properly tracking which members are used. At one time, this was working correctly for buffer_references in GLSL. @spencer-lunarg knows the details here, creating this issue to track it.

[spencer-lunarg]

Taking a look at now, seems this got regressed when we had to add all the various work around to prevent infinite recursion of pointers... I just added some tests locally and can see everything just being UNUSED, looking into fix now

[spencer-lunarg]

Added #298 as well, we didn't actually have a way to detect if the UNUSED on the variable was regressed or not