Prohibit implicit overrides with lesser visibility

A non-abstract super-declaration is allowed to stand as 'an implementation' of
a fake override only if its visibility is not less than visibilities of all
other super-declarations

 #KT-2491 Fixed

compiler/testData/diagnostics/tests/override/kt2491.kt

@@ -0,0 +1,11 @@
+trait T {
+    public fun foo()
+}
+
+open class C {
+    protected fun foo() {}
+}
+
+class <!CANNOT_INFER_VISIBILITY!>D<!> : C(), T
+
+val obj: C = <!CANNOT_INFER_VISIBILITY!>object<!> : C(), T {}