KT-6014 Wrong ABSTRACT_MEMBER_NOT_IMPLEMENTED for toString implemented by delegation

Members declared in interface or overriding members declared in super-interfaces
can be implemented by delegation even if they override members declared in super-class
(NB for interface this can be only 'kotlin.Any').

compiler/testData/diagnostics/tests/override/kt6014.kt

@@ -0,0 +1,17 @@
+interface IBase {
+    override fun toString(): String
+}
+
+interface IDerived : IBase
+
+object BaseImpl : IBase {
+    override fun toString(): String = "A"
+}
+
+object DerivedImpl : IDerived {
+    override fun toString(): String = "A"
+}
+
+class Test1 : IBase by BaseImpl
+
+class Test2 : IDerived by DerivedImpl