[libFuzzer] move the implementation of the fork mode into a separate file

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer@353891 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/FuzzerIO.h b/FuzzerIO.h
index a78d505..9d849be 100644
--- a/FuzzerIO.h
+++ b/FuzzerIO.h
@@ -39,6 +39,8 @@
 // Returns path to a TmpDir.
 std::string TmpDir();
 
+std::string TempPath(const char *Extension);
+
 bool IsInterestingCoverageFile(const std::string &FileName);
 
 void DupAndCloseStderr();