Fix comparisons of char and wchar_t with wxUniChar[Ref].

Fix comparisons of char and wchar_t with wxUniChar[Ref].

Comparisons didn't work correctly in the other direction as they were not
reversed as needed. Fix this by adding wxDEFINE_COMPARISON_REV() macro which
defines comparisons in terms of the reverse operations and use it for both
wxUniChar and wxUniCharRef.

Closes #14547.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72268 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775