[DirectX] Update DXILCBufferAccess pass for explicitly padded types

[bogner]

Currently DXILCBufferAccess only handles dx.Layout types, but llvm/wg-hlsl#311 is a move away from those to types with explicit padding. Handling for explicitly padded types should be vastly simpler than the current logic, and we should be able to remove the various cases of special handling for arrays.