add warning for local type signatures that use the same type variable names as outer type signatures

for a (poor) example,

f :: a -> a
f x = x
   where
     g :: a  --this is (forall x. x) not the 'a' from f's type signature
             -- So, warn about this signature.
     g = undefined

Because it is likely to be confusing, as well as interfering with any possibility of the type variables being considered scoped by default. In fact, this may be a helpful/explanatory message in cases where there will also be a type error due to the type variables not actually being scoped. (although, detecting those cases particularly and giving a recommended solution for how to give such a type signature, would be a more difficult endeavor (what to recommend?))

Trac metadata

Trac field	Value
Version	6.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Operating system	Unknown
Architecture	Unknown