[libFuzzer] introduce an experimental mode -fork=1, where fuzzing happens in a subprocess (still running multiple inputs per process), thus making the fuzzing more resilient to timeouts and OOMs. This is just a skeleton of the code, and some associated refactoring, not a fully working feature yet. 

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@353570 91177308-0d34-0410-b5e6-96231b3b80d8