[DebugInstrRef][1/9] Add fields for instr-ref variable locations

Add a DBG_INSTR_REF instruction and a "debug instruction number" field to
MachineInstr. The two allow variable values to be specified by
identifying where the value is computed, rather than the register it lies
in, like so:

  %0 = fooinst, debug-instr-number 1
  [...]
  DBG_INSTR_REF 1, 0

See the original RFC for motivation:
http://lists.llvm.org/pipermail/llvm-dev/2020-February/139440.html

This patch is NFCI; it only adds fields and other boiler plate.

Differential Revision: https://reviews.llvm.org/D85741