Set the repeatCount to Float.infinity. This compiles and works.
In all probability, HUGE_VALF was a legacy value in any case.
Still, it’s a bit of a surprise that these numeric constant names are not seen by Swift. I did try importing <stdlib.h> in the bridging header but it didn’t help.
But please see now Martin R’s answer. Since the time when I posted my answer, Apple has stated their preferred answer explicitly: use .greatestFiniteMagnitude. It is almost the same thing as .infinity but not quite!