[libc++] Fix uninitialized algorithms when using unconstrained comparison operators (#69373)

If an iterator passed to std::uninitialized_copy & friends provided an
unconstrained comparison operator, we would trigger an ambiguous
overload resolution because we used to compare against
__unreachable_sentinel in our implementation.

This patch fixes that by only comparing the output iterator when it is
actually required, i.e. in the <ranges> versions of the algorithms.

Fixes #69334