libstdc++: Avoid using std::__to_address with iterators

In r12-3935-g82626be2d633a9 I added the partial specialization
std::pointer_traits<__normal_iterator<It, Cont>> so that __to_address
would work with __normal_iterator objects. Soon after that, François
replaced it in r12-6004-g807ad4bc854cae with an overload of __to_address
that served the same purpose, but was less complicated and less wrong.

I now think that both commits were mistakes, and that instead of adding
hacks to make __normal_iterator work with __to_address, we should not be
using __to_address with iterators at all before C++20.

The pre-C++20 std::__to_address function should only be used with
pointer-like types, specifically allocator_traits<A>::pointer types.
Those pointer-like types are guaranteed to be contiguous iterators, so
that getting a raw memory address from them is OK.

For arbitrary iterators, even random access iterators, we don't know
that it's safe to lower the iterator to a pointer e.g. for std::deque
iterators it's not, because (it + n) == (std::to_address(it) + n) only
holds within the same block of the deque's storage.

For C++20, std::to_address does work correctly for contiguous iterators,
including __normal_iterator, and __to_address just calls std::to_address
so also works. But we have to be sure we have an iterator that satisfies
the std::contiguous_iterator concept for it to be safe, and we can't
check that before C++20.

So for pre-C++20 code the correct way to handle iterators that might be
pointers or might be __normal_iterator is to call __niter_base, and if
necessary use is_pointer to check whether __niter_base returned a real
pointer.

We currently have some uses of std::__to_address with iterators where
we've checked that they're either pointers, or __normal_iterator
wrappers around pointers, or satisfy std::contiguous_iterator. But this
seems a little fragile, and it would be better to just use
std::__niter_base for the pointers and __normal_iterator cases, and use
C++20 std::to_address when the C++20 std::contiguous_iterator concept is
satisfied. This patch does that.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string::assign): Replace
	use of __to_address with __niter_base or std::to_address as
	appropriate.
	* include/bits/ptr_traits.h (__to_address): Add comment.
	* include/bits/shared_ptr_base.h (__shared_ptr): Qualify calls
	to __to_address.
	* include/bits/stl_algo.h (find): Replace use of __to_address
	with __niter_base or std::to_address as appropriate. Only use
	either of them when the range is not empty.
	* include/bits/stl_iterator.h (__to_address): Remove overload
	for __normal_iterator.
	* include/debug/safe_iterator.h (__to_address): Remove overload
	for _Safe_iterator.
	* include/std/ranges (views::counted): Replace use of
	__to_address with std::to_address.
	* testsuite/24_iterators/normal_iterator/to_address.cc: Removed.