[libc++][ranges] Fix the return value of `{copy,move}_backward`.

[libc++][ranges] Fix the return value of `{copy,move}_backward`.

The return value for both of these algorithms is specified as
```
`{last, result - N}` for the overloads in namespace `ranges`.
```
But the current implementation instead returns `{first, result - N}`.

Also add both algorithms to the relevant "robust" tests.

Differential Revision: https://reviews.llvm.org/D130968