os: factor out newFileStatFromWin32FileAttributeData

os: factor out newFileStatFromWin32FileAttributeData

The stat function is quite long on Windows. Simplify it a bit by
factoring out the creation of a fileStat from a Win32FileAttributeData.

This also makes it more consistent with the creation of fileStats
from other sources, which all have their own dedicated functions.

Change-Id: I0443f96d892b70ce7f3b5e92c5049e4e4a240c6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/566435
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>