Return valid pointers from wxDateTime::ParseXXX() with non-wxString strings.

Return valid pointers from wxDateTime::ParseXXX() with non-wxString strings.

The pointer returned by wxDateTime::ParseXXX() methods could point into a
buffer of a temporary wxString created to wrap a char* or wchar_t* argument so
dereferencing it was illegal.

Fix this by defining separate overloads for char*/wchar_t* arguments returning
pointers into the original string.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70996 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775