We were failing to tidy the argument coercions of a UnivCo, which
led directly to #25391.

The fix is, happily, trivial.

I don't have a small repro case (it came up when building horde-ad,
which uses typechecker plugins).  It should be possible to make a
repro case, by using a plugin (which builds a UnivCo) but I decided
it was not worth the bother. The bug is egregious and easily fixed.