cgen, checker: allow using smartcasted sumtype variant values in the ORM queries (fix #23239) (#23241)

vlib/orm/orm_sum_type_insert_test.v

@@ -0,0 +1,26 @@
+import db.sqlite
+
+struct SomeStruct {
+	foo int
+	bar string
+}
+
+struct OtherStruct {
+	baz f64
+}
+
+type SomeSum = SomeStruct | OtherStruct
+
+fn test_sum_type_insert() {
+	db := sqlite.connect(':memory:')!
+	sql db {
+		create table SomeStruct
+	}!
+
+	some := SomeSum(SomeStruct{})
+	if some is SomeStruct {
+		sql db {
+			insert some into SomeStruct
+		}!
+	}
+}