Greedy set cover implementation of `Merger::Merge`

Extend the existing single-pass algorithm for `Merger::Merge` with an algorithm that gives better results. This new implementation can be used with a new **set_cover_merge=1** flag.

This greedy set cover implementation gives a substantially smaller final corpus (40%-80% less testcases) while preserving the same features/coverage. At the same time, the execution time penalty is not that significant (+50% for ~1M corpus files and far less for smaller corpora). These results were obtained by comparing several targets with varying size corpora.

Change `Merger::CrashResistantMergeInternalStep` to collect all features from each file and not just unique ones. This is needed for the set cover algorithm to work correctly. The implementation of the algorithm in `Merger::SetCoverMerge` uses a bitvector to store features that are covered by a file while performing the pass. Collisions while indexing the bitvector are ignored similarly to the fuzzer.

Reviewed By: morehouse

Differential Revision:

GitOrigin-RevId: e6597dbae84034ce8ef97802584039b723adb526
7 files changed