Use kFunctionCid instead of kIllegalCid to indicate that a generic function

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 77b87e63a9a96b35a1d2df54549771b3dfa958f6..7b973fa4446b168d400970179955c9263149d7a4 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -6057,7 +6057,7 @@ class TypeParameter : public AbstractType {
     return raw_ptr()->parameterized_function_;
   }
   bool IsClassTypeParameter() const {
-    return parameterized_class_id() != kIllegalCid;
+    return parameterized_class_id() != kFunctionCid;
   }
   bool IsFunctionTypeParameter() const {
     return parameterized_function() != Function::null();