[libFuzzer] introduce an experimental mode -fork=1, where fuzzing happens in a...

[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. 

llvm-svn: 353570