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

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

This avoids overwriting tail padding when algorithms like std::copy are
used to write a single value through a pointer to a base subobject.

The pointer arithmetic on a Base* is valid for N==1, but the copy/move
operation needs to be done using assignment, not a memmove or memcpy of
sizeof(Base) bytes.

Instead of putting a check for N==1 in all of copy, copy_n, move etc.
this adds it to the __copy_move and __copy_move_backward partial
specializations used for trivially copyable types. When N==1 those
partial specializations dispatch to new static member functions of the
partial specializations for non-trivial types, so that a copy/move
assignment is done appropriately for the _IsMove constant.

libstdc++-v3/ChangeLog:

	PR libstdc++/108846
	* include/bits/stl_algobase.h (__copy_move<false, false, RA>)
	Add __assign_one static member function.
	(__copy_move<true, false, RA>): Likewise.
	(__copy_move<IsMove, true, RA>): Do not use memmove for a single
	value.
	(__copy_move_backward<IsMove, true, RA>): Likewise.
	* testsuite/25_algorithms/copy/108846.cc: New test.
	* testsuite/25_algorithms/copy_backward/108846.cc: New test.
	* testsuite/25_algorithms/copy_n/108846.cc: New test.
	* testsuite/25_algorithms/move/108846.cc: New test.
	* testsuite/25_algorithms/move_backward/108846.cc: New test.

(cherry picked from commit 822a11a1e642e0abe92a996e7033a5066905a447)