Do not support opaque pointers in type skeletons for now

See #1877 for further discussion.

src/SAWScript/Crucible/LLVM/Skeleton.hs

@@ -98,6 +98,13 @@ makeLenses ''ModuleSkeleton
 
 parseType :: LLVM.Type -> IO TypeSkeleton
 parseType (LLVM.PtrTo t) = pure $ TypeSkeleton t True [SizeGuess 1 True "default guess of size 1"]
+-- It is unclear how to combine opaque pointers with type skeletons due to the
+-- lack of a pointee type. For now, we simply fail if we encounter one
+-- (see #1877).
+parseType LLVM.PtrOpaque = fail $ unlines
+  [ "Skeleton generation does not support opaque pointers."
+  , "You should report this issue at: https://github.com/GaloisInc/saw-script/issues/1877"
+  ]
 parseType (LLVM.Array i t) = pure $ TypeSkeleton t True
   [ SizeGuess (fromIntegral i) True $ "default guess of size " <> Text.pack (show i)
   ]