Add save/load methods for LLVMPointsToSet

[ga0]

Is your feature request related to a problem? Please describe.
LLVMPointsToSet takes too much time.

Describe the solution you'd like
Since parallelize LLVMPointsToSet is too hard ( #305 ), providing methods to serialize/deserialize or save/load LLVMPointsToSet may be very useful.

    ProjectIRDB IRDB({path});
    LLVMPointsToSet PTS(IRDB, false, PointerAnalysisType::CFLAnders);
    std::ofstream PointsToSetCacheFile("/tmp/points_to_set");
    PTS.save(PointsToSetCacheFile);

Next time when we want to analyze the same project.

    ProjectIRDB IRDB({path});
    LLVMPointsToSet PTS(IRDB);
    std::ifstream PointsToSetCacheFile("/tmp/points_to_set");
    PTS.load(PointsToSetCacheFile);

This will save much time.

The PointsToSetMap is an unordered_map of llvm::Value* pointers.
I don't known how to serializing these pointers' relations.
Can you give me some instructions?

[fabianbs96]

Updates are in #441