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

#KT-4763 Fixed

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 {}