LLDB needs to change the virtual current working directory without it
breaking fallthrough. Most reproducers will contain a virtual working
directory that does not exist in the real file system during replay.
Currently, calling setCurrentWorkingDirectory on RedirectingFileSystem
will attempt to change the working directory for the external FS and if
that fails, it will set ExternalFSValidWD to false which prevents
fallthrough.
I think this could use a comment to explain its behaviour since it's not obvious.