This allows them to be used in constexpr expressions, such as the following:

```
constexpr int distance = absl::c_distance(std::array<int, 3>());
```

Requires at least C++17 to be constexpr.

PiperOrigin-RevId: 648435141
Change-Id: I8136e351a6dc4c25f06ef895fb449f4f11048480