Offer an API to provide the IL mapping information

[xusheng6]

Internally we have a mapping between different layers of IL, which enables us to do the selection sync across different panes. This information is also useful for several other cases, e.g., when the user selects an call instruction at the disassembly level and wish to do forward type propagation, I will need to first find the HLIL instruction corresponding to the selected disassembly instruction.