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

[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.

llvm-svn: 237043