libstdc++: Fix regression in std::move algorithm (PR 93872)

libstdc++: Fix regression in std::move algorithm (PR 93872)

The std::move and std::move_backward algorithms dispatch to the
std::__memmove helper when appropriate. That function uses a
pointer-to-const for the source values, preventing them from being
moved. The two callers of that function have the same problem.

Rather than altering __memmove and its callers to work with const or
non-const source pointers, this takes a more conservative approach of
casting away the const at the point where we want to do a move
assignment. This relies on the fact that we only use __memmove when the
type is trivially copyable, so we know the move assignment doesn't alter
the source anyway.

	PR libstdc++/93872
	* include/bits/stl_algobase.h (__memmove): Cast away const before
	doing move assignment.
	* testsuite/25_algorithms/move/93872.cc: New test.
	* testsuite/25_algorithms/move_backward/93872.cc: New test.