[libc++] Fix _CopySegment helper in ranges::copy(join_view, out) when called...

[libc++] Fix _CopySegment helper in ranges::copy(join_view, out) when called in a static assertion context (#69593)

Resolves Issue #69083

The `_CopySegment` helper for `ranges::copy(join_view, out)` is not
`constexpr` causing rejection in `libc++` in a static assertion context
as in the issue snippet.