[DirectX backend] add support ConstantBuffer to DXILResource.h

class ConstantBuffer is added to save information for cbuffer.
Also add CBufferDataLayout to calculate the size for cbuffer.

Now always use legacy cbuffer layout.
https://reviews.llvm.org/D134998 will add control to disable legacy cbuffer layout.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D136031