[libc++][NFC] Reduce the memory footprint of __copy_cv a bit (#87718)

Instead of instantiating `__copy_cv` for every combination of `_From`
and `_To` this only instantiates `__copy_cv` for every `_From` type,
reducing the number of instantiations.

GitOrigin-RevId: b63fe0d72e2df3b3c4b9fcb91aea07b2582be195
1 file changed