It doesn't matter.
The point of this exchange is to show us how well they know and understand each other. We, the viewers, don't know the Doctor well enough to know what he was going to say, but we can appreciate that Clara does, and that this is the final goodbye between two people who are so close to each other that words are hardly needed between them.
That said, I imagine what the Doctor would have said to Clara would be something along the lines of how much he cared for her, how much he'd enjoyed travelling with her, and how sorry he was that this had to happen. Remember this quote from the end of Series 8, when they thought they weren't going to travel together any more:
CLARA: Doctor? Travelling with you made me feel really special. Thank you for that. Thank you for making me feel special.
DOCTOR: Thank you for exactly the same.
-- Series 8 Episode 12, Death in Heaven
It would have been something along these lines - how special she'd made him feel, how much he loved her - along with his terrible sorrow and regret at her having to die.