Konstantin Varlamov f537a01d39 [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
2022-08-02 22:22:59 -07:00
..