libstdc++: Fix <ext/pointer.h> incompatibilities with C++20

libstdc++: Fix <ext/pointer.h> incompatibilities with C++20

The equality operators for _ExtPtr_allocator are defined as non-const
member functions, which causes ambiguities in C++20 due to the
synthesized operator!= candidates. They should always have been const.

The _Pointer_adapter class template has both value_type and element_type
members, which makes readable_traits<_Pointer_adapter<T>> ambiguous. The
intended workaround is to add a specialization of readable_traits.

	* include/ext/extptr_allocator.h (_ExtPtr_allocator::operator==)
	(_ExtPtr_allocator::operator!=): Add missing const qualifiers.
	* include/ext/pointer.h (readable_traits<_Pointer_adapter<S>>): Add
	partial specialization to disambiguate the two constrained
	specializations.

From-SVN: r280067