os: treat non-symlink reparse points as irregular files

Prior to this change (as of CL 143578), our stat function attempted to
resolve all reparse points as if they were symlinks.

This results in an additional call to CreateFile when statting a
symlink file: we use CreateFile once to obtain the reparse tag and
check whether the file is actually a symlink, and if it is we call
CreateFile again without FILE_FLAG_OPEN_REPARSE_POINT to stat the link
target. Fortunately, since symlinks are rare on Windows that overhead
shouldn't be a big deal in practice.

Fixes #42919.

Change-Id: If453930c6e98040cd6525ac4aea60a84498c9579
Reviewed-on: https://go-review.googlesource.com/c/go/+/460595
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

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