Contexts on empty data decls falsely rejected

[ksf]

HList uses EmptyDataDecls? and contains

data HNat x => Label x ns desc 

which fails with

Data/HList/Label3.hs:37:0:
    No context is allowed on a GADT-style data declaration
    (You can put a context on each contructor, though.) 

, regressing from 6.10.

My crystal ball tells me someone has been simplifying the implementation of EmptyDataDecls? to be GADTs without where, and missed the corner case.

[batterseapower]

Duplicate of #3850