Consistently handle DST start time in wxDateTime::Set().

Always move the dates invalid due to DST (i.e. falling into the "missing" hour
on the DST start date) forward, as GNU libc does, even when using a different
CRT implementation, such as MSVC one which moves the invalid dates backwards.

This seems more expected and also fixes an especially bad problem which
happened due to moving the date backwards in Brazilian time zone where DST
starts at midnight as doing this changed the day and totally broke ParseDate()
assumption that setting wxDateTime to 00:00:00 at the given date really did
set it to this date.

Closes #15419.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74777 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775