libstdc++: Fix minor bugs in std::common_iterator

libstdc++: Fix minor bugs in std::common_iterator

The noexcept-specifier for some std::common_iterator constructors was
incorrectly using an rvalue as the first argument of
std::is_nothrow_assignable_v. This gave the wrong answer for some types,
e.g. std::common_iterator<int*, S>, because an rvalue of scalar type
cannot be assigned to.

Also fix the friend declaration to use the same constraints as on the
definition of the class template. G++ fails to diagnose this error, due
to PR c++/96830.

Finally, the copy constructor was using std::move for its argument
in some cases, which should be removed.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (common_iterator): Fix incorrect
	uses of is_nothrow_assignable_v. Fix inconsistent constraints on
	friend declaration. Do not move argument in copy constructor.
	* testsuite/24_iterators/common_iterator/1.cc: Check for
	noexcept constructibnle/assignable.