[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 command line.

Patch By: Kode Williams

Differntial Revision: https://reviews.llvm.org/D48054

llvm-svn: 336597
GitOrigin-RevId: d153d46884efd6fd4feea6f5b268efa7c6c5573a
6 files changed