VirtualFileSystem: Fix a few directory traversal bugs in VFSWriter

There are a couple of issues with writing VFS maps that are awkward to
fix within the current mutually recursive approach. Instead, replace
the algorithm with an iterative version that uses an explicit stack of
directories.

Includes tests for cases the old approach was tripping on.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209332 91177308-0d34-0410-b5e6-96231b3b80d8