Create high level LLVM nop or coreclr intrinsic for debug info

[michellemcdaniel]

We need an actual high level LLVM nop or other type of LLVM intrinsic for telling LLVM that we need the debug info attached to the instruction to make it through to when the object file is written. We currently insert an inline asm instruction, which isn't exactly the right thing to do and is architecture dependent. We need something in LLVM that we can use that will be architecture independent and will either convert to a nop when we lower or will pass its debug information on to any following instructions that don't have debug info.