Use empty types for TTG extension constructors

I happened to be looking at AmbiguousFieldOcc and noticed that it has gained an XAmbiguousFieldOcc extension constructor for Trees That Grow, but lots of the destructor functions in HsTypes panic if they see this constructor (which is understandable, because they aren't expecting extensions).

In general, perhaps it should be the case that for concrete phase descriptors (e.g. GhcRn) where extension constructors are not expected, the extension constructor argument type should be empty? It would then be clear that they should not be expected (barring abuse of laziness) and pattern matching on them could eliminate the empty type.

That is, instead of the existing

data NoExt = NoExt

type instance XXAmbiguousFieldOcc (GhcPass _) = NoExt

we would define

data NoExtConstructor -- empty data type

noExtConstructor :: NoExtConstructor -> a
noExtConstructor x = case x of {}

type instance XXAmbiguousFieldOcc (GhcPass _) = NoExtConstructor

and similarly for other XX... type families.

Alan, Shayan, is there any reason this couldn't work?

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Trac field	Value
Version	8.5
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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