Change the way directory iteration flags are constructed.

Change the way directory iteration flags are constructed.

Instead of explicitly constructing the flags from the flags that should be
included, construct them by excluding the flags that shouldn't be used. This
makes the code more stable in the sense that it will continue to work when new
flags, such as the upcoming wxDIR_NO_FOLLOW, are added.

See #14542.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72739 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775