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 {

compiler/testData/diagnostics/tests/override/FakeOverrideDifferentDeclarationSignatures.kt

@@ -0,0 +1,11 @@
+trait A {
+    fun f(): String = "string"
+}
+
+open class B {
+    open fun f(): CharSequence = "charSequence"
+}
+
+class <!MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>C<!> : B(), A
+
+val obj: A = <!MANY_IMPL_MEMBER_NOT_IMPLEMENTED!>object<!> : B(), A {}

compiler/testData/diagnostics/tests/override/kt4763.kt

@@ -0,0 +1,11 @@
+trait A {
+    fun f(): String
+}
+
+open class B {
+    open fun f(): CharSequence = "charSequence"
+}
+
+class <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>C<!> : B(), A
+
+val d: A = <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object<!> : B(), A {}

compiler/testData/diagnostics/tests/override/kt4763property.kt

@@ -0,0 +1,11 @@
+trait P {
+    var f: Number
+}
+
+open class Q {
+    val x: Int = 42
+}
+
+class <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>R<!> : P, Q()
+
+val s: Q = <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>object<!> : Q(), P {}