Cast arguments to proper types when generating delegates

Also minor refactoring in a couple places to adopt this style

compiler/testData/codegen/classes/delegationGenericLongArg.kt

@@ -0,0 +1,12 @@
+trait A<T, U> {
+    fun foo(t: T, u: U): String
+}
+
+class Derived(a: A<Long, Int>) : A<Long, Int> by a
+
+fun box(): String {
+    val o = object : A<Long, Int> {
+        override fun foo(t: Long, u: Int) = if (t == u.toLong()) "OK" else "Fail $t $u"
+    }
+    return Derived(o).foo(42, 42)
+}