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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238078 91177308-0d34-0410-b5e6-96231b3b80d8