[libFuzzer] Initial implementation of weighted mutation leveraging during runtime.

Summary:
Added functions that calculate stats while fuzz targets are running and give
mutations weight based on how much new coverage they provide, and choose better
performing mutations more often.

Patch by Kodé Williams (@kodewilliams).

Reviewers: Dor1s, metzman, morehouse

Reviewed By: Dor1s, morehouse

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

Differential Revision: https://reviews.llvm.org/D49621

llvm-svn: 338776