libstdc++: Add constructors and assignments for tuple<> with tuple-like types [PR119721]

libstdc++: Add constructors and assignments for tuple<> with tuple-like types [PR119721]

This patch adds support for constructing and assigning tuple<> from
other empty tuple-like types (e.g., array<T, 0>), completing the C++23
tuple-like interface for the zero-element tuple specialization.

The implementation includes:
- Constructor from forwarding reference to tuple-like types
- Allocator-aware constructor from tuple-like types
- Assignment operator from tuple-like types
- Const assignment operator from tuple-like types

	PR libstdc++/119721

libstdc++-v3/ChangeLog:

	* include/std/tuple (tuple<>::tuple(const tuple&))
	(tuple<>::operator=(const tuple&)): Define as defaulted.
	(tuple<>::swap): Moved the defintion after assignments.
	(tuple<>::tuple(_UTuple&&))
	(tuple<>::tuple(allocator_arg_t, const _Alloc&, _UTuple&&))
	(tuple<>::operator=(_UTuple&&)) [__cpp_lib_tuple_like]: Define.
	(tuple<>::operator==, tuple<>::opeator<=>): Parenthesize
	constrains individually.
	* testsuite/23_containers/tuple/cons/119721.cc: New test for
	constructors and assignments with empty tuple-like types.
	* testsuite/20_util/tuple/requirements/empty_trivial.cc:
	New test verifying tuple<> remains trivially copyable.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>