[MLIR] Erase location of folded constants (#75415)

Follow up to the discussion from #75258, and serves as an alternate
solution for #74670.

Set the location to Unknown for deduplicated / moved / materialized
constants by OperationFolder. This makes sure that the folded constants
don't end up with an arbitrary location of one of the original ops that
became it, and that hoisted ops don't confuse the stepping order.