More precise LANGUAGE pragma when forall is used

In all cases when a user attempts to use the forall keyword, GHC suggests enabling RankNTypes. This seems suboptimal, since a more direct way to enable just the syntax is ExplicitForAll. So maybe we should just suggest that instead.

In some cases, the user will immediately thereafter have to add RankNTypes, or Rank2Types, or ExistentialQuantification, etc. So it would be more user friendly to suggest the appropriate extension directly. I don't know how much harder this would be to do.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
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