Fix wxULongLong::ToDouble() for MSVC.

Fix wxULongLong::ToDouble() for MSVC.

The conversion of wxULongLong to double was broken for MSVC by r40658 which
was supposed to correct some unspecified compilation problem but this code
seems to compile fine with at least recent versions of MSVC and, in any case,
correcting compilation only for the code to work incorrectly during run-time
doesn't look like a good idea.

This fixes current unit test failures in wxMSW.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67634 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775