Add a warning for pattern synonyms without signatures

It is accepted as good style that top-level definitions should have type signatures. Pattern synonyms seem no different to me in this regard so I think it would be good if GHC warned if there was a pattern synonym definition which lacked a type signature.

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Trac field	Value
Version	7.10.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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mentioned in issue #9911 (closed)

changed weight to 5

added Tfeature request Trac import newcomer labels

assigned to @mpickering

mentioned in commit 1883afb2

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	- → patsyns/T11053

Note that including this in -Wall makes it harder for clients to maintain a warning-free library over 3 releases, given that pattern synonyms were available one cycle before their signatures. I'm still not personally entirely convinced about that policy, but it does seem to have traction and we should keep it in mind as we evolve the language.

changed milestone to %8.0.1

added Pnormal label