[libFuzzer] add two experimental flags to make corpus merging more scalable: -save_coverage_summary/-load_coverage_summary. This is still WIP, the documentation will come later if these flags survive

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298548 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed