Suggest -XExistentialQuantification for 'forall' in data declarations

D5180 introduced a slight regression to the error messages. In this code

data T = forall a. MkT a

GHC used to complain

rnfail053.hs:5:10:
    Not a data constructor: ‘forall’
    Perhaps you intended to use ExistentialQuantification

but then the message has become

rnfail053.hs:5:18: error:
    Illegal symbol '.' in type
    Perhaps you intended to use RankNTypes or a similar language
    extension to enable explicit-forall syntax: forall <tvs>. <type>

Trac metadata

Trac field	Value
Version	8.6.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
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