P2278R4 `cbegin` Should Always Return A Constant Iterator

[P2278R4](https://wg21.link/P2278R4) `cbegin` Should Always Return A Constant Iterator

Feature-test macro (expected):
```cpp
#define __cpp_lib_ranges_as_const 202207L
```
