add WARNING pragma

It might be nice to have a WARNING pragma in place of or in addition to the DEPRECATED pragma that just allows you to issue a generic warning:

{-# WARNING f, C, T "These are deprecated" #-}
{-# WARNING g "This has terrible performance" #-}
{-# WARNING T "We had to expose this type for bad reasons, don't use it." #-}
{-# WARNING preConf "This is an unstable interface. May become deprecated." #-}
{-# WARNING fromJust "Calls the error function.  use maybe for safety" #-}
module Wibble {-# WARNING "Use Wibble_1 instead" #-}

This is more general than the DEPRECATED pragma.

Trac metadata

Trac field	Value
Version	6.4.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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