[libFuzzer] Mutation tracking and logging implemented.
Code now exists to track number of mutations that are used in fuzzing in total
and ones that produce new coverage. The stats are currently being dumped to the
Patch by Kodé Williams (@kodewilliams).
Reviewers: metzman, Dor1s, morehouse, kcc
Reviewed By: Dor1s, morehouse, kcc
Subscribers: delcypher, kubamracek, kcc, morehouse, llvm-commits, #sanitizers, mgorny
Differential Revision: https://reviews.llvm.org/D48054