libFuzzer integration

This only works on Linux!

Running fuzzer

bazel --config=asan-libfuzzer //experimental/protobuf:lldb_eval_fuzzer_run

# or

bazel --config=asan-libfuzzer //experimental/fuzzed_rng:lldb_eval_fuzzer_run

Use --config=libfuzzer instead of --config=asan-libfuzzer to disable address sanitizer.

Troubleshooting

If you're changing structure of proto messages, you might encounter ERRORs when running fuzzer. Try to clear /tmp/fuzzing folder where corpus and artifacts of previous runs were saved.