Kind-check CUSK associated types separately
Previously, we kind-checked associated types while while still
figuring out the kind of a CUSK class. This caused trouble, as
documented in Note [Don't process associated types in kcLHsQTyVars]
in TcTyClsDecls. This commit moves this process after the initial
kind of the class is determined.
Fixes Trac #15142.
Test case: indexed-types/should_compile/T15142.hs
(cherry picked from commit 030211d21207dabb7a4bf21cc9af6fa5eb066db1)