Make a way to tell GHC that a pragma name should be "recognised"

GHC warns about unrecognised pragmas, but other impls can add pragmas inbetween GHC releases. If you want to use those pragmas then you can't just use -Wall -Werror when compiling. We could make a pragma to tell GHC to not warn about other pragmas, e.g.

{-# KNOWN_PRAGMA FOO #-}

would make GHC not warn about FOO pragmas.

Suggested in #2847 (closed).

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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