[IRSim] Adding basic implementation of llvm-sim.
This is a similarity visualization tool that accepts a Module and
passes it to the IRSimilarityIdentifier. The resulting SimilarityGroups
are output in a JSON file.
Tests are found in test/tools/llvm-sim and check for the file not found,
a bad module, and that the JSON is created correctly.
Reviewers: paquette, jroelofs, MaskRay
Recommit of: 15645d044bcfe2a0f63156048b302f997a717688 to fix linking
errors and GN build system.
Differential Revision: https://reviews.llvm.org/D86974