K2: prefer derived class sources for callable copies (e.g. fake overrides)

compiler/testData/ir/sourceRanges/declarations/fakeOverrides2.kt

@@ -0,0 +1,7 @@
+abstract class A<T> {
+    fun foo(x: T) {} // (1)
+}
+
+class B : A<String>() {
+    // fake-override fun foo(x: String) // (2) should use 'B' class source
+}