Intellect devourer is not an object
It is an aberration of tiny size.
Nothing states, that they are one and the same entity.
Note, that it is possible to save the host. Furthermore rules state about Intellect devourer leaving the host, when he drops to 0 hp. The host and the Intellect Devourer are two seperate creatures.
How will polymorph work?
While inside a creature, the intellect devourer has total cover against attacks and other effects originating outside its host.
You cannot affect the intellect devourer itself, but only the host.
True polymorph
Choose one creature or nonmagical object that you
can see within range.[...]
The only thing, you will see is once again, the host, not the intellect devourer. Accordingly, to the spell's decription, only one creature will be affected and that will be the host.
What will be the effect?
Despite the fact of being polymorphed, the host is still the same creature.
The effect will be a dog with the intellect devourer inside it's skull. Because True Polymorph usually affects mental ability scores it is possible to notice, that something is wrong, as the dog will have an Intelligence of 12, instead of 3.
Specific Beats General - A permanently polymorphed creature will stay morphed if it dies
In 5e, you always apply a general rule unless there is a specific exception. That is exactly what is happening in this case. Generally, the target will revert after the full duration, upon the target reaching 0 hp, or upon dying. However, there is a specific exception for when the caster concentrated on the spell for the whole duration. In that case, the change lasts until dispelled, full stop. The first sentence no longer applies in this case, because the second is an exception to it. As a result even if the conditions in the first sentence are met, the creature stays transformed because of the second sentence.
"Until the target drops to 0 hit points..." isn't enough because of effects that can instantly kill a creature without affecting hit points. These effects are usually high level magic like the 9th level Power Word: Kill and the 7th level Divine Word both of which can instantly kill a creature without changing their hit points at all, as long as the creature's current hit points are within a certain threshold. In these cases, you do not drop to 0 hp; instead you just immediately die.
Best Answer
Yes, and no.
The simulacrum's statistics are replaced. This means you take whatever form of record sheet you're using for the simulacrum, put it aside, and grab the stat block of the creature you've polymorphed it into and use that while the polymorph effect persists.
While polymorphed, the creature is a tyrannosaur, a giant ape, or what have you. For the duration of the polymorph effect, the simulacrum effectively does not exist, so none of it's restrictions apply to the new replacement.
A true polymorphed creature can regain hit points through resting (polymorph doesn't last long enough to complete a short rest to use hit dice, let alone a long rest). Using true polymorph also lets you potentially pick something with regeneration, a feature not found on any beast I'm aware of.
When the polymorph effect expires and the creature resumes being a simulacrum, it has however many hit points it had before the polymorph. Of course, that's how it works for any other creature, too.