[VirtualFileSystem] Support virtual working directory in the RedirectingFS

[VirtualFileSystem] Support virtual working directory in the  RedirectingFS

Before this patch, changing the working directory of the RedirectingFS
would just forward to its external file system. This prevented us from
having a working directory that only existed in the VFS mapping.

This patch adds support for a virtual working directory in the
RedirectingFileSystem. It now keeps track of its own WD in addition to
updating the WD of the external file system. This ensures that we can
still fall through for relative paths.

This change was originally motivated by the reproducer infrastructure in
LLDB where we want to deal transparently with relative paths.

Differential revision: https://reviews.llvm.org/D65677

llvm-svn: 374917