[libc++][hardening] Check bounds on arithmetic in __bounded_iter (#78876)

Previously, `__bounded_iter` only checked `operator*`. It allowed the
pointer to go out of bounds with `operator++`, etc., and relied on
`operator*` (which checked `begin <= current < end`) to handle
everything. This has several unfortunate consequences:

First, pointer arithmetic is UB if it goes out of bounds. So by the time
`operator*` checks, it may be too late and the optimizer may have done
something bad. Checking both operations is safer.

Second, `std::copy` and friends currently bypass bounded iterator
checks. I think the only hope we have to fix this is to key on `iter +
n` doing a check. See #78771 for further discussion. Note this PR is not
sufficient to fix this. It adds the output bounds check, but ends up
doing it after the `memmove`, which is too late.

Finally, doing these checks is actually *more* optimizable. See #78829,
which is fixed by this PR. Keeping the iterator always in bounds means
`opera...