libstdc++: Implement std::pair/tuple/misc enhancements from P2321R2

This implements the non-<ranges> changes from P2321R2, which primarily
consist of additional converting constructors, assignment operator and
swap overloads for std::pair and std::tuple.

libstdc++-v3/ChangeLog:

	* include/bits/stl_bvector.h (_Bit_reference::operator=): Define
	const overload for C++23 as per P2321R2.
	* include/bits/stl_pair.h (pair::swap): Likewise.
	(pair::pair): Define additional converting constructors for
	C++23 as per P2321R2.
	(pair::operator=): Define const overloads for C++23 as per
	P2321R2.
	(swap): Define overload taking const pair& for C++23 as per
	P2321R2.
	(basic_common_reference): Define partial specialization for
	pair for C++23 as per P2321R2.
	(common_type): Likewise.
	* include/bits/uses_allocator_args.h
	(uses_allocator_construction_args): Define additional pair
	overloads for C++23 as per P2321R2.
	* include/std/tuple (_Tuple_impl::_Tuple_impl): Define
	additional converting constructors for C++23 as per P2321R2.
	(_Tuple_impl::_M_assign): Define const overloads for C++23
	as per P2321R2.
	(_Tuple_impl::_M_swap): Likewise.
	(tuple::__constructible): Define as a convenient renaming of
	_TCC<true>::__constructible.
	(tuple::__convertible): As above but for _TCC<true>::__convertible.
	(tuple::tuple): Define additional converting constructors for
	C++23 as per P2321R2.
	(tuple::operator=): Define const overloads for C++23 as per
	P2321R2.
	(tuple::swap): Likewise.
	(basic_common_reference): Define partial specialization for
	tuple for C++23 as per P2321R2.
	(common_type): Likewise.
	* testsuite/20_util/pair/p2321r2.cc: New test.
	* testsuite/20_util/tuple/p2321r2.cc: New test.
	* testsuite/23_containers/vector/bool/element_access/1.cc: New test.

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