Improve error message for type rigidity

When saying that a GADT match is given a non-rigid type, we should be more explicit about where a type signature would help. For example, given the program:

data E x = E x

data Foo a where
  Foo :: Gadt a -> Foo a

data Gadt a where
  GadtValue :: a -> Gadt (E a)

g = case undefined of
        Foo GadtValue -> ()

I get the error message:

Test.hs:13:12:
    GADT pattern match with non-rigid result type `t'
      Solution: add a type signature
    In a case alternative: Foo GadtValue -> ()
    In the expression: case undefined of { Foo GadtValue -> () }
    In the definition of `g':
        g = case undefined of { Foo GadtValue -> () }

But where should the type signature be? A better hint would be:

Solution: add a type signature, probably at _TYPE_
(case undefined of { Foo GadtValue -> () }) :: _TYPE_

Similarly, we know when it's the scrutinee that needs a signature. And in a do-binding the "scrutinee" is really the right-hand side. We can get all this information from the matching context.

Trac metadata

Trac field	Value
Version	6.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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