Add -fwan-sum-type-partial-field-accessor

This a a new warning flag that would be off by default and warns when a sum type is defined with field accessors that are partial. For an example of something that would trigger the warning:

data Option a
  = Some { getSome :: a }
  | None

In production quality Haskell code it usually desirable to outright ban partial functions like head, fromJust etc by use of a custom prelude. Unfortunately partial functions can still sneak into a code base via field accessors in sum types.

Trac metadata

Trac field	Value
Version	8.4.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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