Bugfix: The compiler were not inserting proper downcasts for generic types

This resulted in a codegen error in C++ mode, because the generic
types were not defined in modules where calls requiring downcasts
were used (generating a downcast forces the inclusion of the full
definition of the involved types).

tests/generics/module_with_generics.nim

@@ -0,0 +1,14 @@
+type
+  Base[T] = ref object {.inheritable.}
+    value*: T
+
+  Derived[T] = ref object of Base[T]
+    derivedValue*: T
+
+proc makeDerived*[T](v: T): Derived[T] =
+  new result
+  result.value = v
+
+proc setBaseValue*[T](a: Base[T], value: T) =
+  a.value = value
+