As I recall Melisande "manufactured" a Lightbringer to push her narrative. The sword was Stannis's but I don't believe it was the original Lightbringer. I'm guessing Brienne has the sword if anybody but I don't think it will fulfill the prophecy. What would be badass cool is if it was in Lyana's tomb along with the evidence necessary to authenticate Jon as a Targaryian/Stark thus wrapping up one story line (Jon's heritage) and providing a device to combat the NK (whatever sword is in there, Lightbringer or Dawn).
Speaking of Dawn, some interesting information regarding it's whereabouts & a quote from GRRM
https://awoiaf.westeros.org/index.php/Dawn