PSI2IR: Fix delegated members generation

When generating bodies for members implemented by delegation, invoke
corresponding delegate member, not an interface member. Otherwise we
might lose platform-specific nullability information in case of mixed
Kotlin-Java hierarchies, as in
implicitNotNullOnDelegatedImplementation.kt

compiler/testData/ir/irText/classes/implicitNotNullOnDelegatedImplementation.kt

@@ -0,0 +1,47 @@
+// FILE: implicitNotNullOnDelegatedImplementation.kt
+interface IFoo {
+    fun foo(): String
+}
+
+class K1 : JFoo()
+
+class K2 : JFoo() {
+    override fun foo() = super.foo()
+}
+
+class K3 : JUnrelatedFoo(), IFoo
+
+class K4 : JUnrelatedFoo(), IFoo {
+    override fun foo() = super.foo()
+}
+
+class TestJFoo : IFoo by JFoo() {
+    // nullability assertion in 'foo()'
+}
+
+class TestK1 : IFoo by K1() {
+    // nullability assertion in 'foo()'
+}
+
+class TestK2 : IFoo by K2() {
+    // no nullability assertion in 'foo()'
+}
+
+class TestK3 : IFoo by K3() {
+    // no nullability assertion in 'foo()'
+}
+
+class TestK4 : IFoo by K4() {
+    // nullability assertion in 'foo()'
+}
+
+
+// FILE: JFoo.java
+public class JFoo implements IFoo {
+    public String foo() { return null; }
+}
+
+// FILE: JUnrelatedFoo.java
+public class JUnrelatedFoo {
+    public String foo() { return null; }
+}