Cast arguments to proper types when generating delegates

Also minor refactoring in a couple places to adopt this style

compiler/testData/codegen/classes/delegationGenericArg.kt

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