Add conversions to/from long long to wxUniChar.

Add conversions to/from long long to wxUniChar.

Allow conversions to/from long long and unsigned long long values in wxUniChar
for consistency with the other integral types.

Also make the code shorter by using helper wxDO_FOR_INT_TYPES() and
wxDO_FOR_CHAR_INT_TYPES() macros to avoid duplicating the same code for all of
the integral types and having to handle wchar_t (and wxLongLong_t now)
specially because sometimes we may need to overload on it and sometimes not.

Finally, add more tests to check that all the wxUniChar methods compile and
work with all the different types.

Closes #15206.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74029 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775