[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

llvm-svn: 355019