Pragma for suppressing 'orphan instance' warnings per instance

Additionally to #2515 (closed) I'd like to suppress orphan instances warnings individually. I do not always like to disable orphan instance warnings globally for a module, since some of the warnings may point to an instance that can actually be moved to a more appropriate module.

Syntax could be

{-# NOWARN orphan #-}
instance C T where
   ...

or simply

{-# NOWARN #-}
instance C T where
   ...

(Before GHC-6.12 a general local NOWARN pragma could also help to minimize wrong warnings about unused imported modules. And sometimes I also like to disable 'unused identifier' warning for some top-level functions, that shall be type checked, but not exported, because their API is still not mature.)

Trac metadata

Trac field	Value
Version	6.12.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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CC	ghc@henning-thielemann.de
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