os: follow all name surrogate reparse points in Stat on Windows
Previously, os.Stat only followed IO_REPARSE_TAG_SYMLINK
and IO_REPARSE_TAG_MOUNT_POINT reparse points.
This CL generalize the logic to detect which reparse points to follow
by using the reparse tag value to determine whether the reparse point
refers to another named entity, as documented in
https://learn.microsoft.com/en-us/windows/win32/fileio/reparse-point-tags.
The new behavior adds implicit support for correctly stat-ing reparse
points other than mount points and symlinks, e.g.,
IO_REPARSE_TAG_WCI_LINK and IO_REPARSE_TAG_IIS_CACHE.