Kind-check CUSK associated types separately

Authored by goldfire on Jul 5 2018, 2:09 PM.

Description

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)