[llvm-reduce] Add pass that reduces DebugInfo metadata

This new pass for llvm-reduce attempts to reduce DebugInfo metadata.
The process used is:
  1. Scan every MD node, keeping track of nodes already visited.
  2. Look for DebugInfo nodes, then record any operands that are lists.
  3. Bisect though all the elements of the collected lists.

Differential Revision: https://reviews.llvm.org/D132077