Fix -Wterm-variable-capture scope (#23434)

-Wterm-variable-capture wasn't accordant with type variable
scoping in associated types, in type classes. For example,
this code produced the warning:
  k = 12

  class C k a where
    type AT a :: k -> Type

I solved this issue by reusing machinery of newTyVarNameRn function
that is accordand with associated types: it does lookup for each free type
variable when we are in the type class context. And in this patch I
use result of this work to make sure that -Wterm-variable-capture warns
only on implicitly quantified type variables.