[libFuzzer][Windows] Port fork mode to Windows

Summary:
Port libFuzzer's fork mode to Windows.
Implement Windows versions of MkDir, RmDir, and IterateDirRecursive to do this.
Don't print error messages under new normal uses of FileSize (on a non-existent file).
Implement portable way of piping output to /dev/null.
Fix test for Windows and comment fork-sigusr.test on why it won't be ported to Win.

Reviewers: zturner

Reviewed By: zturner

Subscribers: kcc, zturner, jdoerfert, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D58513

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer@355019 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/FuzzerMerge.cpp b/FuzzerMerge.cpp
index d70aa3e..dace45e 100644
--- a/FuzzerMerge.cpp
+++ b/FuzzerMerge.cpp
@@ -331,7 +331,7 @@
     Cmd.addFlag("merge_control_file", CFPath);
     Cmd.addFlag("merge_inner", "1");
     if (!V) {
-      Cmd.setOutputFile("/dev/null");  // TODO: need to handle this on Windows?
+      Cmd.setOutputFile(getDevNull());
       Cmd.combineOutAndErr();
     }
     auto ExitCode = ExecuteCommand(Cmd);