[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

Differential Revision: https://reviews.llvm.org/D86974