[libc++] Forward more often to memmove in copy

In D122982 I accidentally disabled the memmove optimization. This re-enables it and adds more cases where copy forwards to memmove.
Fixes https://github.com/llvm/llvm-project/issues/33687

Reviewed By: var-const, #libc, ldionne

Spies: pkasting, ayzhao, dcheng, xbolva00, libcxx-commits

Differential Revision: https://reviews.llvm.org/D124328