[libc++] `std::ranges::advance`: avoid unneeded bounds checks when advancing iterator (#84126)

Currently, the bounds check in `std::ranges::advance(it, n, s)` is done
_before_ `n` is checked. This results in one extra, unneeded bounds
check.

Thus, `std::ranges::advance(it, 1, s)` currently is _not_ simply
equivalent to:

```c++
if (it != s) {
    ++it;
}
```

This difference in behavior matters when the check involves some
"expensive" logic. For example, the `==` operator of
`std::istreambuf_iterator` may actually have to read the underlying
`streambuf`.

Swapping around the checks in the `while` results in the expected
behavior.

0 files changed, 0 insertions, 0 deletions