[lib/Fuzzer] start getting rid of std::cerr. Sadly, these parts of C++ library...

[lib/Fuzzer] start getting rid of std::cerr. Sadly, these parts of C++ library used in libFuzzer badly interract with the same code used in the target function and also with dfsan. It's easier to just not use std::cerr than to defeat these issues. 

llvm-svn: 238078