[Fuzzer] Optimize UpdateFeatureFrequency (#65288)

Instead of a linear scan, use a bitset to track rarity of features. This
improves fuzzer throughput rather dramatically (close to 2x) in early
exploratory phases; in steady state this seems to improve fuzzing
throughput by ~15% according to perf.

The benchmarks are done on an executable with ~100k features, so the
results may change based on the executable that's being fuzzed.

kFeatureSetSize is 2M so the bitset is adding 256 KB to
sizeof(InputCorpus), but this should be fine since there's already three
arrays indexed by feature index for a total of 200 MB.