[llvm][vfs] Preserve paths for fallback/fallthrough in RedirectingFileSystem (#85307)

When we lookup in the external filesystem, do not remove . and ..
components from the original path. For .. this is a correctness issue in
the presence of symlinks, while for . it is simply better practice to
preserve the original path to better match the behaviour of other
filesystems. The only modification we need is to apply the working
directory, since it could differ from the external filesystem.

rdar://123655660