libstdc++: Implement const copy-assignment for tuple<> [PR119721]

libstdc++: Implement const copy-assignment for tuple<> [PR119721]

This patch completes the implementation of P2321R2, giving tuple proper proxy
reference semantics.

The assignment operator is implemented as a template constrained to accept only
tuple<>. Consequently, the language does not consider it a copy assignment
operator, which prevents tuple<> from losing its trivially copyable status.

The _Tuple template parameter is defaulted, ensuring the operator remains
a viable candidate for assignment with an empty brace-init list.

	PR libstdc++/119721

libstdc++-v3/ChangeLog:

	* include/std/tuple (tuple<>::operator=(const _Tuple&) const)
	[__cpp_lib_ranges_zip]: Define.
	* testsuite/23_containers/tuple/cons/119721.cc: Test const
	assignment.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>