ForEachInst optionally runs on attached debug line insts

ForEachInst optionally runs on attached debug line insts

Also:
- Add const forms of ForEachInst
- Rewrite Module::ToBinary in terms of ForEachInst
- Add Instruction::ToBinaryWithoutAttachedDebugInsts
- Delete the ToBinary method on Function, BasicBlock, and Instruction
  since it can now be implemented with ForEachInst in a less confusing
  way, e.g. without recursion.
- Preserve debug line instructions on OpFunctionEnd (and store that
  instruction as a unique-pointer, for regularity).