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/t5602_inheritence.nim

@@ -1,10 +1,11 @@
 discard """
   output: "seq[float]\n0"
+  targets: "c cpp"
 """
 
 # https://github.com/nim-lang/Nim/issues/5602
 
-import typetraits
+import typetraits, module_with_generics
 
 type
   Foo[T] = object of RootObj
@@ -16,3 +17,8 @@ proc p[T](f: Foo[T]): T =
 var s: Bar[float]
 echo p(s).len # the bug was: p(s) should return seq[float], but returns float instead
 
+# Test overloading and code generation when
+# downcasting is required for generic types:
+var d = makeDerived(10)
+setBaseValue(d, 20)
+