The Eldritch Invocation "Aspect of the Moon" from Xanathar's Guide to Everything (pg. 56) provides you with the benefit of no longer needing to sleep:

Aspect of the Moon

Prerequisite: Pact of the Tome feature

You no longer need to sleep and can't be forced to sleep by any means. To gain the benefits of a long rest, you can spend all 8 hours doing light activity, such as reading your Book of Shadows and keeping watch.

Elves do not sleep; they "trance" (from the PHB, pg. 23):

Trance

Elves don’t need to sleep. Instead, they meditate deeply, remaining semiconscious, for 4 hours a day. (The Common word for such meditation is “trance.”) While meditating, you can dream after a fashion; such dreams are actually mental exercises that have become reflexive through years of practice. After resting in this way, you gain the same benefit that a human does from 8 hours of sleep.

If I have an elven warlock, does gaining the Aspect of the Moon invocation override the need to trance? Can an elven warlock still trance if they want to (since elves can gain the benefits of a long rest in 4h if they trance, but presumably not if they don't trance as per Aspect of the Moon)? Or is it the case that, because trancing is not sleeping, this invocation does nothing for an elf?

The elf can choose to use Trance or Aspect of the Moon

The language from the Invocation says (emphasis mine)

can spend all 8 hours...

This suggests that it is a choice to utilize that versus the standard long rest option for the character.

The Functional Differences

Aspect of the Moon gives you the option to be awake and aware for the full 8 hours to gain the benefits of a long rest.

Trance gives you the option to shut down for just 4 hours for in order to gain the benefits of a Long Rest - but you are not aware for those 4 hours while in the trance. See the printed Sage Advice for reference to this change:

If an elf meditates during a long rest (as described
in the Trance trait), the elf finishes the rest after only 4
hours. A meditating elf otherwise follows all the rules for
a long rest; only the duration is changed.

Yes, they can still Trance

Aspect of the Moon doesn't eliminate the ability for a character to sleep, it simply eliminates their need to sleep, or any ability for them to be forced to sleep, like with the Sleep spell.

It would similarly stand to reason that even if there were an interaction between Elven "Trancing" and the effects of Aspect of the Moon, no part of Aspect of the Moon would prevent the elf from Trancing instead of staying awake the whole duration, should they choose to do so.

Elves with the Aspect of the Moon do not need to trance.

Both the Aspect of the Moon and Trance give you ways to spend a long rest, and both are options in addition to the standard rules for a long rest, as neither one says that you can't sleep. Each time you take a long rest, you choose which option you want to use.

Elves still require 8 hours of rest to gain benefits of a long rest. They just don't require 8 hours of sleep.
http://dnd.wizards.com/articles/features/rules-answers-september-2015