Recent CL 41834 made windows Stat work for all symlinks.
But CL 41834 also made Stat slow.

John Starks sugested
(see https://github.com/golang/go/issues/19922#issuecomment-300031421)
to use GetFileAttributesEx for files and directories instead.
This makes Stat as fast as at go1.9.

I see these improvements on my Windows 7

name       old time/op  new time/op  delta
StatDot    26.5µs ± 1%  20.6µs ± 2%  -22.37%  (p=0.000 n=9+10)
StatFile   22.8µs ± 2%   6.2µs ± 1%  -72.69%  (p=0.000 n=10+10)
StatDir    21.0µs ± 2%   6.1µs ± 3%  -71.12%  (p=0.000 n=10+9)
LstatDot   20.1µs ± 1%  20.7µs ± 6%   +3.37%  (p=0.000 n=9+10)
LstatFile  6.23µs ± 1%  6.36µs ± 8%     ~     (p=0.587 n=9+10)
LstatDir   6.10µs ± 0%  6.14µs ± 4%     ~     (p=0.590 n=9+10)

and on my Windows XP

name         old time/op  new time/op  delta
StatDot-2    20.6µs ± 0%  10.8µs ± 0%  -47.44%  (p=0.000 n=10+10)
StatFile-2   20.2µs ± 0%   7.9µs ± 0%  -60.91%  (p=0.000 n=8+10)
StatDir-2    19.3µs ± 0%   7.6µs ± 0%  -60.51%  (p=0.000 n=10+9)
LstatDot-2   10.8µs ± 0%  10.8µs ± 0%   -0.48%  (p=0.000 n=10+8)
LstatFile-2  7.83µs ± 0%  7.83µs ± 0%     ~     (p=0.844 n=10+8)
LstatDir-2   7.59µs ± 0%  7.56µs ± 0%   -0.46%  (p=0.000 n=10+10)

Updates #19922

Change-Id: Ice1fb5825defb05c79bab4dec0692e0fd1bcfcd5
Reviewed-on: https://go-review.googlesource.com/43071Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>