libstdc++: Do not use use memmove for 1-element ranges [PR108846,PR116471]

libstdc++: Do not use use memmove for 1-element ranges [PR108846,PR116471]

This commit ports the fixes already applied by r13-6372-g822a11a1e642e0
to the range-based versions of copy/move algorithms.

When doing so, a further bug (PR116471) was discovered in the
implementation of the range-based algorithms: although the algorithms
are already constrained by the indirectly_copyable/movable concepts,
there was a failing static_assert in the memmove path.

This static_assert checked that iterator's value type was assignable by
using the is_copy_assignable (move) type traits. However, this is a
problem, because the traits are too strict when checking for constness;
a type like

  struct S { S& operator=(S &) = default; };

is trivially copyable (and thus could benefit of the memmove path),
but it does not satisfy is_copy_assignable because the operator takes
by non-const reference.

Now, the reason for the check to be there is because a type with
a deleted assignment operator like

  struct E { E& operator=(const E&) = delete; };

is still trivially copyable, but not assignable. We don't want
algorithms like std::ranges::copy to compile because they end up
selecting the memmove path, "ignoring" the fact that E isn't even
copy assignable.

But the static_assert isn't needed here any longer: as noted before,
the ranges algorithms already have the appropriate constraints; and
even if they didn't, there's now a non-discarded codepath to deal with
ranges of length 1 where there is an explicit assignment operation.

Therefore, this commit removes it. (In fact, r13-6372-g822a11a1e642e0
removed the same static_assert from the non-ranges algorithms.)

libstdc++-v3/ChangeLog:

	PR libstdc++/108846
	PR libstdc++/116471
	* include/bits/ranges_algobase.h (__assign_one): New helper
	function.
	(__copy_or_move): Remove a spurious static_assert; use
	__assign_one for memcpyable ranges of length 1.
	(__copy_or_move_backward): Likewise.
	* testsuite/25_algorithms/copy/108846.cc: Extend to range-based
	algorithms, and cover both memcpyable and non-memcpyable
	cases.
	* testsuite/25_algorithms/copy_backward/108846.cc: Likewise.
	* testsuite/25_algorithms/copy_n/108846.cc: Likewise.
	* testsuite/25_algorithms/move/108846.cc: Likewise.
	* testsuite/25_algorithms/move_backward/108846.cc: Likewise.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>