libstdc++: Apply the move_iterator changes described in P1207R4

libstdc++: Apply the move_iterator changes described in P1207R4

These changes are needed for some of the tests in the constrained algorithm
patch, because they use move_iterator with an uncopyable output_iterator.  The
other changes described in the paper are already applied, it seems.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (move_iterator::move_iterator): Move __i
	when initializing _M_current.
	(move_iterator::base): Split into two overloads differing in
	ref-qualifiers as in P1207R4 for C++20.