[lib/Fuzzer] add a trace-based mutatation logic. Same idea as with DFSan-based mutato...

[lib/Fuzzer] add a trace-based mutatation logic. Same idea as with DFSan-based mutator, but instead of relying on taint  tracking, try to find the data directly in the input. More (logic and comments) to go.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237043 91177308-0d34-0410-b5e6-96231b3b80d8