[DebugInstrRef] Parse debug instruction-references from/to MIR

This patch defines the MIR format for debug instruction references: it's an
integer trailing an instruction, marked out by "debug-instr-number", much
like how "debug-location" identifies the DebugLoc metadata of an
instruction. The instruction number is stored directly in a MachineInstr.

Actually referring to an instruction comes in a later patch, but is done
using one of these instruction numbers.

I've added a round-trip test and two verifier checks: that we don't label
meta-instructions as generating values, and that there are no duplicates.

Differential Revision: https://reviews.llvm.org/D85746