Note [TyClGroups and dependency analysis]

A TyClGroup represents a strongly connected component of type/class/instance
decls, together with the role annotations and standalone kind signatures for the
type/class declarations.

The hs_tyclds :: [TyClGroup] field of a HsGroup is a dependency-order
sequence of strongly-connected components.

Invariants
 * The type and class declarations, group_tyclds, may lexically depend
   on each other, or earlier TyClGroups, but not on later ones

 * The role annotations, group_roles, are role-annotations for some or
   all of the types and classes in group_tyclds (only).

 * The instance declarations, group_instds, may (and usually will)
   lexically depend on group_tyclds, or on earlier TyClGroups, but
   not on later ones.

See Note [Dependency analysis of type and class decls] in GHC.Rename.Module
for more info.