[libFuzzer] make LLVMFuzzerTestOneInput (the fuzzer target function) return int inste...

[libFuzzer] make LLVMFuzzerTestOneInput (the fuzzer target function) return int instead of void. The actual return value is not *yet* used (and expected to be 0). This change is API breaking, so the fuzzers will need to be updated.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249214 91177308-0d34-0410-b5e6-96231b3b80d8