Add warning for initializers with obsolete syntax

Expected 'init' keyword before class initializer

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/kt244.kt

@@ -14,7 +14,7 @@ fun f(s: String?) {
 // more tests
 class A(a: String?) {
     val b = if (a != null) <!DEBUG_INFO_SMARTCAST!>a<!>.length() else 1
-    {
+    init {
         if (a != null) {
             val <!UNUSED_VARIABLE!>c<!> = <!DEBUG_INFO_SMARTCAST!>a<!>.length()
         }
@@ -22,7 +22,7 @@ class A(a: String?) {
 
     val i : Int
 
-    {
+    init {
         if (a is String) {
             i = <!DEBUG_INFO_SMARTCAST!>a<!>.length()
         }