Use memmove() instead of memcpy() in wxString::AssignCopy().

Use memmove() instead of memcpy() in wxString::AssignCopy().

This at least allows the code like "s = s.c_str()" to work correctly when
using our own wxString implementation, even it doesn't fix all
self-assignment-related bugs (again, when using our own implementation only,
there is no bug when using std::basic_string as underlying implementation).

This is a cherry pick of r63008 from 2.8 branch.

See #11245.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70150 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775