[MIRPrinter] Add machine metadata support.

- Distinct metadata needs generating in the codegen to attach correct
  AAInfo on the loads/stores after lowering, merging, and other relevant
  transformations.
- This patch adds 'MachhineModuleSlotTracker' to help assign slot
  numbers to these newly generated unnamed metadata nodes.
- To help 'MachhineModuleSlotTracker' track machine metadata, the
  original 'SlotTracker' is rebased from 'AbstractSlotTrackerStorage',
  which provides basic interfaces to create/retrive metadata slots. In
  addition, once LLVM IR is processsed, additional hooks are also
  introduced to help collect machine metadata and assign them slot
  numbers.
- Finally, if there is any such machine metadata, 'MIRPrinter' outputs
  an additional 'machineMetadataNodes' field containing all the
  definition of those nodes.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D103205