ELV lampholders do not have anything built in. There are ELV light fittings that have a transformer "built in", if by built in you mean attached to the same bit of metal that screws onto the ceiling. Even then they don't have rectifiers because they've never needed them. Are there newer fittings intended for LEDs with the whole circuitry built in to rectify AC? Maybe, but I haven't seen them.
Most LED ELV lamps have rectifiers built in, yes all in that tiny shell, so that they are plug-in replacements and can run off AC. Some don't and require an external driver of some sort that provides DC, for example a SMPS, and obviously can't be used as plug-in replacements for halogen lamps in most fittings.
Since most LED lamps accept AC input and rectify it themselves, it doesn't matter which way you plug them in. Those which expect DC presumably do need to be plugged in the correct way. Has anyone got one of these rare beasties handy? I've got some LED strips that have an obvious polarity, but not a more conventional light bulb.