libstdc++: Implement P2538R1 ADL-proof std::projected

This was recently approved for C++26, but there's no harm in
implementing it unconditionally for C++20 and C++23. As it says in the
paper, it doesn't change the meaning of any valid code. It only enables
things that were previously ill-formed for questionable reasons.

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (projected): Replace class
	template with alias template denoting an ADL-proofed helper.
	(incremental_traits<projected<Iter, Proj>>): Remove.
	* testsuite/24_iterators/indirect_callable/projected-adl.cc:
	New test.