[libFuzzer] extend the -fork=1 functionality. Still not fully usable, but good enough for the first unit test

llvm-svn: 353775
diff --git a/compiler-rt/lib/fuzzer/FuzzerLoop.cpp b/compiler-rt/lib/fuzzer/FuzzerLoop.cpp
index c3be3dd..5cd0cdd 100644
--- a/compiler-rt/lib/fuzzer/FuzzerLoop.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerLoop.cpp
@@ -537,15 +537,16 @@
   delete[] DataCopy;
 }
 
-void Fuzzer::WriteToOutputCorpus(const Unit &U) {
+std::string Fuzzer::WriteToOutputCorpus(const Unit &U) {
   if (Options.OnlyASCII)
     assert(IsASCII(U));
   if (Options.OutputCorpus.empty())
-    return;
+    return "";
   std::string Path = DirPlusFile(Options.OutputCorpus, Hash(U));
   WriteToFile(U, Path);
   if (Options.Verbosity >= 2)
     Printf("Written %zd bytes to %s\n", U.size(), Path.c_str());
+  return Path;
 }
 
 void Fuzzer::WriteUnitToFileWithPrefix(const Unit &U, const char *Prefix) {