Re-add wxFD_OVERWRITE_PROMPT and wxFD_FILE_MUST_EXIST to wxGenericFileDialog.

Support for these styles was accidentally removed in r48733, re-add them back.

Closes #16652.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78080 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775