Add toString() to Any, fix all tests

#KT-4517 Fixed

compiler/testData/diagnostics/tests/Builders.kt

@@ -52,7 +52,7 @@ fun main(args : Array<String>) {
 trait Element {
   fun render(builder : StringBuilder, indent : String)
 
-  fun toString() : String? {
+  override fun toString() : String {
     val builder = StringBuilder()
     render(builder, "")
     return builder.toString()

compiler/testData/diagnostics/tests/infos/Autocasts.kt

@@ -183,7 +183,7 @@ fun returnFunctionLiteral(a: Any?): Function0<Int> =
 fun mergeAutocasts(a: Any?) {
   if (a is String || a is Int) {
     a.<!UNRESOLVED_REFERENCE!>compareTo<!>("")
-    a.toString()
+    <!DEBUG_INFO_AUTOCAST!>a<!>.toString()
   }
   if (a is Int || a is String) {
     a.<!UNRESOLVED_REFERENCE!>compareTo<!>("")