[libFuzzer] Set -experimental_len_control=1000 as default.

[morehouse]

Summary

Experiments using
https://github.com/google/fuzzer-test-suite/tree/master/engine-comparison
show a significant increase in coverage and reduction in corpus size
with this option enabled.

Addresses https://llvm.org/pr36371.

[morehouse]

• Rename flag from experimental_len_control to len_control.

[kcc]

LGTM

compiler-rt/lib/fuzzer/FuzzerFlags.def

 //===----------------------------------------------------------------------===//
 FUZZER_FLAG_INT(verbosity, 1, "Verbosity level.")
 FUZZER_FLAG_UNSIGNED(seed, 0, "Random seed. If 0, seed is generated.")
 FUZZER_FLAG_INT(runs, -1,
             "Number of individual test runs (-1 for infinite runs).")
 FUZZER_FLAG_INT(max_len, 0, "Maximum length of the test input. "
     "If 0, libFuzzer tries to guess a good value based on the corpus "
     "and reports it. ")
-FUZZER_FLAG_INT(experimental_len_control, 0, "experimental flag")
+FUZZER_FLAG_INT(experimental_len_control, 1000, "experimental flag")
 FUZZER_FLAG_INT(cross_over, 1, "If 1, cross over inputs.")
 FUZZER_FLAG_INT(mutate_depth, 5,
             "Apply this number of consecutive mutations to each input.")
 FUZZER_FLAG_INT(reduce_depth, 0, "Experimental/internal. "
                 "Reduce depth if mutations lose unique features")
 FUZZER_FLAG_INT(shuffle, 1, "Shuffle inputs at startup")
 FUZZER_FLAG_INT(prefer_small, 1,
     "If 1, always prefer smaller inputs during the corpus shuffle.")