Don't allow fake override to inherit implementation with wrong return type

#KT-4763 Fixed

compiler/testData/codegen/box/classes/selfcreate.kt

@@ -1,7 +1,7 @@
 class B () {}
 
 open class A(val b : B) {
-    fun a() = object: A(b) {}
+    fun a(): A = object: A(b) {}
 }
 
 fun box() : String {

compiler/testData/codegen/bytecodeText/bridgeForFakeOverride.kt

@@ -1,5 +1,5 @@
 trait A {
-    fun foo(): Object
+    fun foo(): Any
 }
 
 open class B {