os: do not report ModeDir for symlinks on windows

os: do not report ModeDir for symlinks on windows

When using Lstat against symlinks that point to a directory,
the function returns FileInfo with both ModeDir and ModeSymlink set.
Change that to never set ModeDir if ModeSymlink is set.

Fixes #10424
Fixes #17540
Fixes #17541

Change-Id: Iba280888aad108360b8c1f18180a24493fe7ad2b
Reviewed-on: https://go-review.googlesource.com/41830
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>