libstdc++: Make std::pointer_traits SFINAE-friendly [PR96416]

This implements the resolution I'm proposing for LWG 3545, to avoid hard
errors when using std::to_address for types that make pointer_traits
ill-formed.

Consistent with std::iterator_traits, instantiating std::pointer_traits
for a non-pointer type will be well-formed, but give an empty type with
no member types. This avoids the problematic cases for std::to_address.
Additionally, the pointer_to member is now only declared when the
element type is not cv void (and for C++20, when the function body would
be well-formed). The rebind member was already SFINAE-friendly in our
implementation.

libstdc++-v3/ChangeLog:

	PR libstdc++/96416
	* include/bits/ptr_traits.h (pointer_traits): Reimplement to be
	SFINAE-friendly (LWG 3545).
	* testsuite/20_util/pointer_traits/lwg3545.cc: New test.
	* testsuite/20_util/to_address/1_neg.cc: Adjust dg-error line.
	* testsuite/20_util/to_address/lwg3545.cc: New test.