[libFuzzer] clear the corpus elements if they are evicted (i.e. smaller elements...

[libFuzzer] clear the corpus elements if they are evicted (i.e. smaller elements with proper coverage are found). Make sure we never try to mutate empty element. Print the corpus size in bytes in the status lines

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283279 91177308-0d34-0410-b5e6-96231b3b80d8