[HLSL] Treat structures in resources as packed

[bogner]

Structures in HLSL's StructuredBuffer need to be treated as packed, or otherwise made to skip adding any padding for compatibility with DXC.

Note: dxc does this by hacking padding off of all structures in CGRecordLowering::lower, like so:

#if 0 // HLSL Change - No padding for structure. Array offset will be handled when load/store is called
  Members.push_back(StorageInfo(Size, getIntNType(8)));
  clipTailPadding();
  determinePacked(NVBaseType);
  insertPadding();
  Members.pop_back();
  calculateZeroInit();
#endif // HLSL Change End

This is obviously not how we should handle this in upstream.