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

#KT-4763 Fixed

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 {}