[libFuzzer] Implement stat::stability_rate based on the percentage of unstable edges.

Summary:
Created a -print_unstable_stats flag.
When -print_unstable_stats=1, we run it 2 more times on interesting inputs poisoning unstable edges in an array.
On program termination, we run PrintUnstableStats() which will print a line with a stability percentage like AFL does.

Patch by Kyungtak Woo (@kevinwkt).

Reviewers: metzman, Dor1s, kcc, morehouse

Reviewed By: metzman, Dor1s, morehouse

Subscribers: delcypher, llvm-commits, #sanitizers, kcc, morehouse, Dor1s

Differential Revision: https://reviews.llvm.org/D49212

llvm-svn: 337175